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HARVARD MEDICAL
LIBRARY
RONTGEN
THE LLOYD E. HAWES
COLLECTION IN THE
HISTORY OF RADIOLOGY
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THE INTEKPRETATION OF RADIUM
AND THE STRUCTURE OF
THE ATOM
THE INTERPRETATION
OF RADIUM
AND THE STRUCTURE OF THE ATOM
BY
FREDERICK SODDY, M.A., F.R.S.
DR. lee's professor OF INORGANIC AND PHYSICAL CHEMISTRY, UNIVERSITY OF OXFORD
WITH ILLUSTRATIONS
FOURTH EDITION
REVISED AND ENLARGED
G. P. PUTNAM'S SONS
NEW YORK
1922
First Edition March, 1909
Second Edition --.--. November, 1909,
Third Edition October, 1912.
Fourth Edition ------ August, 1920.
Beprinted - - - -, - - - . May, 1922.
PREFACE TO THE FOURTH EDITION
In again revising this book I have conformed to. the
eariier plan of writing what I should have said if
the lectures had been delivered in 1920 instead of
1908. The original statement has been amplified
rather than modified. It has lost, long since, the
appearance of challenge to existing theories which at
first it may have presented.
But the subject has now grown entirely beyond
the power of being fully encompassed by the original
very simple and popular mode of treatment. I have
thought it best, therefore, not to compress the original
part unduly, as it still may serve a useful purpose
to those not familiar with scientific conceptions, but
to add, as a second part, a more briefly written and
less elementary account of the later developments,
particularly those that bear upon the problem of
the constitution of the atom. It is to be hoped
that even those who are not chemists or physicists,
who have followed the exposition in the first part,
may not be entirely unable to profit by the second
part. Though, naturally, the new subject-matter,
by reasons of its more general and often more specu-
lative character — much of it still being in the making
— cannot but be more difficult to understand than the
original work, which dealt with distinct and easily
understood steps in the progress of knowledge, made
once and for all time.
FREDERICK SODDY.
The Uniy£bsitt of Oxford,
July., 1920
PREFACE
The present-day interpretation of radium, that it is
an element undergoing spontaneous disintegration,
was put forward in a series of joint scientific com-
munications to the Philosophical Magazine of 1902
and 1903 by Professor Rutherford, now of Manchester
University, and myself. As its application is not
confined to the physical sciences, but has a wide and
general bearing on our whole outlook upon Nature, I
have attempted in this book a presentation of the
subject in non- technical language, so that the ideas
involved, and their bearing upon current thought, may
be within the reach of the lay reader. Although written
in non-technical language, no effort has been spared to
get to the root of the matter and to secure accuracy, so
that possibly the book may prove serviceable to workers
in other fields of science and investigation as well as
to the general public.
The book contains the main substance of six popular
experimental lectures delivered in the University of
Glasgow at the beginning of the year, but being relieved
from the necessity, always present in lecturing, of co-
ordinating the experimental and descriptive sides, I
have, while adhering to the lecture form of address,
entirely rearranged and very largely rewritten the
subject matter in order to secure the greatest possible
degree of continuity of treatment. Certain portions
of the lectures, for example those dealing with the
X-rays and the spectra of elements, have been omitted,
and attention thereby concentrated upon radium, the
chief topic. In addition, I have briefly embodied the
viii PREFACE
results of important discoveries which have appeared
since the date of the lectures, particularly the experi-
ments of Professor Rutherford and Dr. Geiger in count-
ing the number of a-particles expelled by radium. The
book also contains some account of the arrangement
by means of which I have recently succeeded in
detecting and measuring the quantity of the helium
generated from the common radio-elements uranium
and thorium.
I have borrowed freely from numerous scattered
lectures and addresses bearing on the subject which
I have from time to time been invited to deliver, and
may mention in particular the Wilde lecture to the
Manchester Literary and Philosophical Society, 1904,
the Presidential and other addresses to the Rontgen
Society, 1906, the opening of the discussion on the
evolution of the elements in Section A of the British
Association Meeting in York, 1906, and the Watt
lecture to the Greenock Philosophical Society. 1908.
FREDERICK SODDY.
The University, Glasgow,
November, 1908.
CONTENTS
PART I
CHAPTER I
PAOB
THE DISCOVERY OF KADIOACTIVITY
Radioactivity, a new science — Its discovery — The four experi-
mental effects of radioactivity — The rays of radioactive sub-
stances— The continuous emission of energy from the radio-
elements - - - - - - -1
CHAPTER II
RADIUM
Radioactivity an unalterable atomic property — The radioactivity
of thorium — Pitchblende — Quantity of radium in pitchblende —
The smallest quantity of radium detectable — Experiments with
radium — Cost of radium — The doctrine of energy — Measure-
ment of the energy emitted by radium — The source of cosmical
energy — Radium and the " physically impossible " - - 12
CHAPTER III
THE RAYS OF RADIOACTIVE SUBSTANCES
The radiations of the radio-elements — a-, j3-, and y-rays — Test of
penetrating power — Experiments with the penetrating p- and
y-rays — The feebly penetrating a-rays — Experiment with a-
rays — The range of a-rays in air — The physical nature of
radiation — Corpuscular radiation — The wave theory of hght —
a- and j3- rays due to the expulsion of particles — The individual
atom of matter — The spinthariscope — The decay of a-radiation
— Counting the a-particles - - - - - 28
X CONTENTS
CHAPTER IV
PAGE
THE RAYS OF RADIOACTIVE SUBSTANCES (COntmUed)
The |3-rays — Deviation by a magnet — Electric charge carried by
^-rays — The nature of electricity — Radiant matter or cathode-
rays — The electron — Inertia or mass — Velocity of the /S-rays —
The radium clock — Magnetic deviation of the a-particle — Its
velocity — Passage of a-particles through matter — Scattering
of a-particles — Method of rendering the track of rays visible —
The fates of a-particles - - - - - - 47
CHAPTER V
THE RADIUM EMANATION
The source of radioactive energy — Two alternative theories — ^The
internal energy of matter — Radium a changing element —
Disintegration in cascade — The successive outbursts of energy
— The radium emanation — Experiments with the emanation —
Its condensation by cold — The infinitesimal quantity of the
emanation — Its radioactivity — The chemical character of the
emanation — The heat evolved by the emanation — ^The decay
of the emanation — Its reproduction by radium — Atomic dis-
integration— Radioactive equilibrium — Energy of radioactive
change — All radioactive changes equally detectable - - 68
CHAPTER VI
HELIUM AND RADIUM
The connection of the a-particle with radioactive changes — Helium
and the a-particle — The ultimate products — Discovery of
helium, solar and terrestrial — Prediction of the production of
helium — Production of helium from radium — Its production
from uranium and thorium — Identity of the a-particle and
helium — The first change of radium — Radioactive recoil - 93
CHAPTER VII
THEORY OF ATOMIC DISINTEGRATION
Questions of nomenclature — Definition of the atom — Elements
and chemical compounds — The experimental facts — The nature
of atomic disintegration — The chance of disintegration —
CONTENTS xi
PAGE
The period of average life of a disintegrating atom — ^The un-
known cause of disintegration — Determination of the period of
average life — The period of average fife of radium — The total
energy evolved in the complete disintegration of radium - 105
CHAPTER VIII
THE OBIGIN OF BADITJM
Why has radium survived ? — The reproduction of radium —
The ratio between the quantities of uranium and radium in all
minerals — Hydraulic analogy to radioactive change — The age of
pitchblende — Uranium X — Attempts to detect a growth of
radium — Existence of intermediate products — Ionium —
Production of radium by uranium — The stately procession of
elementary evolution .---.. 121
CHAPTER IX
THE SUCCESSIVE CHANGES OF RADIUM
The later changes of radium — ^The active deposit of radium — The
radiations from the active deposit — Experiments with the
active deposit — Radium A — Radium B and C — The radiation
from the emanation — The later slow changes of radium —
Radium D, E, and F — Polonium — The ultimate product of
radium — Uranium I, and Uranium II — Uranium X^, and
Uranium X2 (Brevium) — Radium C and Radium C^ - - 136
CHAPTER X
EADIOACTIVITY AND THE NATURE OP MATTER
Ratio of quantities of polonium and radium in minerals — Table
of the ratio of the quantities of all the products of uranium —
Impossibility of concentrating many of the products of dis-
integration— Increase of activity of radium with time — The
rarity of elenients — The currency metals — The nature of atoms
— The velocity of a-particles — StabiUty and survival of ele-
ments— Connection between range of a-rays and period —
Pleochroic halos — Uranium and thorium halos - - - 152
xii CONTENTS
CHAPTER XI
FAOE
RADIOACTIVITY AND THE EVOLUTION OP THE WORLD
The potentialities of matter — Why radium is unique — The total
energy evolved by uranium — The importance of transmutation
— Primitive man and fire — Source of cosmical energy — Radium
in the earth's crust — Various possible fates of the earth — The
most probable view — Radioactivity and mythology — The new
prospect - - - - • - - 168
PART II
CHAPTER XII
THE THORIUM AND ACTINIUM DISINTEGRATION SERIES
The thorium disintegration series — Mesothorium and radiothorium
— Radioactivity of thorium — Mesothorium — ^The thorium
emanation — Radiothorium — Experiments with the thorium
emanation — Thorium A — The actinium disintegration series —
The origin of actinium — Multiple atomic disintegration —
Branch series of thorium and radium — The actinium branch
series — The actinium emanation — Actinium A — Eka-tan-
talum or proto-actinium — Uranium Y — Table of complete dis-
integration series — The unsolved riddle of matter - - 186
CHAPTER XIII
THE ULTIMATE STRUCTURE OF MATTER
A flood of knowledge — ^The nature of mass — Sir J. J. Thomson's
model atom — The periodic law — Electrolytic dissociation —
The outermost region of the atom .... 209
CHAPTER XIV
THE NUCLEAR ATOM
The innermost region of the atom — An artificial transmutation
— Atoms compared and contrasted with solar systems - 220
CONTENTS xiii
CHAPTER XV
PAGE
ISOTOPES
Elements which are chemically identical — The periodic law and
radioactive changes — The atomic number — Isotopic elements —
The problem of the ancient alchemist - - - - 227
CHAPTER XVI
THE X.RAYS AND CONCLUDING EVIDENCE
The X-ray spectra of the elements — The y-rays — The intermediate
region of atomic structure — The homogeneous characteristic
X-rays of Barkla — ^The atomic mass or weight — The element
lead — The separation of isotopes — Neon and Metaneon — The
general prevalence of isotopism — The problem of transmuta-
tion— Conclusion ..,.._ 234
INDEX - - -253
'o face
31
LIST OF ILLUSTRATIONS
Fia,
1. Becquerel's uranium radiograph of an aluminium"!
medallion - - - - - 1
2. Welsbach mantle, taken by the rays from the |
thorium contained in it - - - 1
3- Photograph and radiograph of a piece of pitch-
blende (Sir William Crookes) - . . „ 15
4j Photograph of silk tassel electrified by friction -|
5. The same discharged by the rays of radium - ) "
6. Radium writing on a photographic plate -
7. Box of compasses taken by y-rays of radium
8. Diagram of coated flask and radium-covered dish
for showing a-rays - - - - - - 35
9. Photograph of the same apparatus - -To face 35
10. Diagram of Spinthariscope of Sir William
Crookes - - - - .--43
11. Photograph of the Spinthariscope - - - To face 35
12. Photograph of the electro-magnet for deviating
the /3-rays . - . . - ,,47
13. Diagram of magnetic deviation of /3-rays - - - - 49
14. Diagram of Crookes' tube to show magnetic devia-
tion of cathode-rays - - - - - - 54
15. Diagram of Strutt's radium clock - - - - - 59
16. Photograph of radium clock - - -To face 47
17. Tracks of a-particles photographed by C. T. R
Wilson .....
18. Track of a single a- and of a single /3-particle - /" » 65
19. Tracks of two a-particles — one straight, one twice
deflected - - - -
20. Photograph of tube containing AviJIemite - -\
21. Photograph of the same tube by its own light when r „ 78
containing radium emanation - - --'
22. Diagram of apparatus for showing the condensa-
tion of the radium emanation - - - - - 81
23. Diagram of the first disintegration of radium - - - 94
XVI
LIST OF ILLUSTRATIONS
FIG,
24. Photograph of the spectrum-tube in which the pro-
duction of heUum from radium emanation was
observed . . . . -
25. Photograph by Dr. Giesel of the spectrum of
helium produced from radium
26. Photograph of apparatus for detecting and measur-
ing heUum produced from uranium and thor-
ium ------
27. Diagram showing the first change of radium
28. Diagram for the first disintegration of uranium
29. Diagram of the uranium-radium disintegration
series (initial changes) - - -
30. Diagram of the first four disintegrations of radium
31. Diagram of apparatus for obtaining the active
deposit of radium - . - .
32. Photograph of the same apparatus
33. Diagram of the later disintegrations of radium
34. Diagram of the complete uranium disintegration
series ------
85. Microphotograph of uranium and thorium pleo-]
chroic halos ( Joly) - - - - 1
36. Enlarged photograph of uranium halo showing!
ring due to Radium A - - •)
37. Diagram of the complete thorium disintegration
series ------
38. Diagram of the complete actinium disintegration
series ------
39. The brandling of the thorium series
40. The branching of the radium series
41. Initial part of uranium series showing branch
actinium series . - - -
42. Table showing complete disintegration series
43. The periodic table of the elements -
44. Chart showing a- and /3-change and periodic law
generalisation ....
To face 99
„ 100
- 103
- 129
- 133
- 139
- 141
To face 141
- 146
- 150
To face 166
- 190
- 199
- 201
- 202
- 206
- 207
- 214
- 230
THE INTERPRETATION OF
RADIUM AND THE STRUCTURE
OF THE ATOM
PART I
CHAPTER I
THE DISCOVERY OF RADIOACTIVITY
Radioactivity, a New Science.
One of the main duties of science is the correlation
of phenomena, apparently disconnected and even
contradictory. For example, chemistry teaches us
to regard under one aspect, as various types of
cambustion or oxidation, the burning of a candle,
the rusting of metals, the physiological process of
respiration, and the explosion of gunpowder. In each
process there is the one common fact that oxygen
enters into new chemical combinations. Similarly
to the physicist, the fall of the traditional apple of
Newton, the revolution of the earth and planets round
the sun, the apparitions of comets, and the ebb and
flow of the tides are all phases of the universal law
of gravitation. A race ignorant of the nature of
combustion or of the law of gravitation, and ignorant
of the need of such generalisations, could not be con-
sidered to have advanced far along the paths of
scientific discovery. The phenomena with which I
am concerned in these lectures belong to the newly-
born science of radioactivity and to the spontaneous
disintegration of elements which the study of radio-
2 THE DISCOVERY OF RADIOACTIVITY
activity has revealed to us. It is a natural inquiry
to ask — To what most nearly are these new phe-
nomena correlated ? Is it possible to give, by the
help of an analogy to familiar phenomena, any correct
idea of the nature of this new phenomenon " Radio-
activity " ? The answer may surprise those who
hold to the adage that there is nothing new under the
sun. Frankly, it is not possible, because in these latest
developments science has broken fundamentally new
ground, and has delved one distinct step further down
into the foundations of knowledge.
During the century which has just closed there
occurred, it is true, at an ever-increasing rate, a cease-
less extension of our knowledge of the nature of matter
upon which physical science is largely based. Yet this
advance was for the most part an expansion rather than
a deepening. It was concerned with what may be
termed atomic and molecular architecture, the external
qualities of atoms and the construction and study of
complexes built of atoms — that is to say, molecules.
As buildings are built of bricks, so compounds can
nowadays be built up out of atoms. The atoms are
to the chemist and physicist what bricks are to the
architect — the units supplied ready-made to a certain
limited number of standard specifications and dimen-
sions capable of an endless variety of combinations
and arrangements, each with its own peculiarities and
external relationships.
The century which has just begun has seen the first
definite and considerable step taken into the ultimate
nature of these units of matter or atoms, which is in one
sense not merely an extension of existing knowledge or
principles, but a radically new departure. Radio-
activity is a new primary science owing allegiance
neither to physics nor chemistry, as these sciences were
understood before its advent, because it is concerned
with a knowledge of the elementary atoms themselves
of a character so fundamental and intimate that the
old laws of physics and chemistry, concerned almost
A PERENNIAL SUPPLY OF ENERGY 3
wholly with external relationships, do not suffice.
This first step has indeed emphasised how superficial
our knowledge of matter has really been. If one
were to demonstrate to an architect that the bricks
he habitually and properly employs in his constructions
were under other circumstances capable of entirely
different uses — let us say, for illustration, that they
could with effect be employed as an explosive incom-
parably more powerful in its activities than dynamite —
the surprise of the architect would be no greater than
the surprise of the chemist at the new and undreamt
of possibilities of matter demonstrated by the mere
existence of such an element as radium.
In this first lecture our attention will be mainly
directed to the one outstanding feature in connection
with radium, and the property of radioactivity which
it exhibits to an extraordinary degree, in which the
whole range of its remarkable features are epitomised.
The radioactive substances evolve a perennial supply
of energy from year to year without stimulus and without
exhaustion. It would be idle to deny with regard
to this that physical science was taken completely by
surprise. Had any one twenty-five years ago ventured
to predict radium he would have been told simply
that such a thing was not only wildly improbable,
but actually opposed to all the established principles
of the science of matter and energy. So drastic an
innovation was, it is true, unanticipated. Radium,
however, is an undisputed fact to-day, and there is no
question which would have triumphed in the conflict,
had its existence conflicted with the established prin-
ciples of science. Natural conservatism and dislike of
innovation appear in the ranks of science more strongly
than most people are aware. Indeed, science is no
exception. There was, however, never the slightest
ground for assuming that because the new facts were
startling and unexpected they must necessarily conflict
with older knowledge. That would be to pay science
a poor compliment. Some of the new facts we shall
4 THE DISCOVERY OF RADIOACTIVITY
discuss in the lectures appeared at first, and may-
even yet appear to you, almost incredible, but that is
only on account of the entire newness of the whole
region to which they belong. Into this region the
older chemistry and physics have, as we have seen,
never before penetrated. It is not until we begin
to apply to the new facts the established principles of
science, which have served so well of old, that their
full significance gradually becomes evident. Keep
in mind that our knowledge of Nature is always of
necessity partial, and is bounded in all directions by
certain inevitable but too often forgotten limitations
connected, for example, with the briefness of human
life and the physical impossibility of pursuing investi-
gations except under conditions where the life of
the investigator can be maintained. The laws and
principles of physical science, old and new, are alike
subject to these perpetual limitations, and are neces-
sarily only true within these limits. From this point
of view there is nothing in the many surprising pro-
perties of radium which conflicts with a single estab-
lished principle of older science. Physics and chemistry
remain almost unchanged where they were, and radio-
activity, so far as it is concerned with the correctness
of their principles, has, as a matter of fact, given to
the old laws and theories a fuller and truer signifi-
cance than they had before. The extension of the
old theories which has been rendered necessary has
not been revolutionary in any destructive sense. It
is wonderful how accommodating a true theory is to
new truth, apparently of a diametrically opposite
character, and this not in any sense of mere ingenuity
of explanation, but in a manner that arrests the in-
vestigator, and is his sign that he is on safe ground.
It may seem a paradox, but from the first the best
proof of the newer views, to my mind, was in the com-
pleteness with which the strange, newly-won know-
ledge of radioactivity harmonised with the old views of
the chemist about atoms and elements. On the other
AN EPOCH-MAKING CONCLUSION 5
hand this gratifying harmony, where conflict might
have been expected, is not a surrender. On every
hand new vistas of thought are opening out. We
see the simple and direct answer to many problems
before deemed insoluble. We recognise now causes at
work where before we only saw effects, many of them
so familiar and ingrained in our consciousness that the
necessity for a cause had been almost overlooked, or,
if felt at all, met perfunctorily and wholly inadequately
by existing knowledge. Highly technical and compli-
cated as many of the researches on radioactivity are,
the main conclusions of the science are as simple and
certain as they are fundamental, and of general interest.
It is the duty of every educated man to make himself
aware of the chief bearings of these conclusions, for
they touch human life strangely at many points, and
are destined in the future to influence profoundly the
course of philosophic thought. In a few years the
elementary principles of radioactivity will be taught
in all schools as belonging to the very beginnings of
physical science. To-day, while all is strange and new
and the very name of the science even unfamiliar, it
may appear a far cry to attempt to foretell the effects
these discoveries, concerned primarily with the ultimate
nature of matter, are destined to exert on our concep-
tions of the ultimate destiny of man. But already
the most direct connection is apparent. Indeed, this
aspect of the advance is perhaps the most revolu-
tionary. We shall be able to see more clearly at the
end how this has come about. At present it is suffi-
cient to indicate that radioactivity has introduced a
new conception into the fundamental problems of
existence. By its conclusion that there is imprisoned
in ordinary common matter vast stores of energy,
which ignorance alone at the present time prevents
us from using for the purposes of life, radioactivity
has raised an issue which it is safe to say will mark
an epoch in the progress of thought. With all our
mastery over the powers of Nature we have adhered
6 THE DISCOVERY OF RADIOACTIVITY
to the view that the struggle for existence is a
permanent and necessary condition of hfe. To-day it
appears as though it may well be but a passing phase,
to be altogether abolished in the future as it has to some
extent been mitigated in the past by the unceasing,
and as it now appears, unlimited ascent of man to
knowledge, and through knowledge to physical power
and dominion over Nature.
The Discovery of Radioactivity,
The first discovery of the property we now call
radioactivity was made in the year 1896 by M. Henri
Becquerel in Paris, and, like many other great dis-
coveries, the actual experiment itself owed something
to luck or chance or accident. Looking backward,
however, it appears rather that only the particular
day or month of the discovery was a matter of chance.
The time was just ripe for the event, and it is certain
that its coming could not long have been delayed.
Some slight historical sketch of the conditions preceding
and immediately following the discovery is necessary
before considering wherein lies its great significance.
The memorable discovery of the X-rays by Professor
Rontgen, in 1895, which is known to all, familiarised
scientific workers with a type of radiation able to
traverse objects opaque to light. The X-rays are
themselves invisible to the unaided eye, but are able
to affect the photographic plate. This led to experi-
ments being made in order to see if similar types of
rays were not produced in other ways. As you all
know, certain substances exposed to sunlight shine
afterwards in the dark, and this property, which finds
an application in the manufacture of luminous paint,
is known as phosphorescence or fluorescence. Is
phosphorescent light entirely stopped by opaque
objects ? Or does it in part consist of invisible pene-
trating j-ays like the X-rays ? M. Becquerel wrapped
a photographic plate in black paper and placed on it a
■V!Sfi«s,«*!jS!W»I^4(l^l|^j|Sjjj
Fig. I. — Becquerel's Uranium Picture.
Fig. 2. — Welsbach Mantle imprinted by its Own Rays.
To face p. 7
URANIUM RADIATION 7
phosphorescent substance which was then exposed
to sunhght. By great good fortune M. Becquerel
chose as the particular phosphorescent body a prep-
aration of uranium, and found as the result of the
experiment that the photographic plate beneath the
preparation was darkened. The preparation had given
out rays which, unlike sunlight, were capable of pene-
trating the black paper. It was soon found that these
rays, like the X-rays, even penetrated thin plates
of metal, for when such a thin plate was interposed
between the preparation and the film darkening still
occurred. But one day, the sun being obscured, the
plate and the phosphorescent uranium preparation
upon it were set aside in a dark drawer for some weeks,
and M. Becquerel, wishing to see if any darkening had
occurred without the sunlight, developed the plate as
it was. It was found that darkening had gone on
just as much in the darkness as in the light. Further
experiments soon established that neither sunlight
nor phosphorescence had anything to do with the
experiment. The action is an entirely new inherent
property of the element uranium. No other phosphores-
cent body would have darkened the plate even in the
sunlight, while all preparations containing uranium
do so, whether they are phosphorescent or not, in
total darkness as well as in the light. Fig. 1 shows
one of the photographs by uranium rays obtained by
M. Becquerel. Between the patch of the uranium
preparation and the plate was placed an aluminium
medallion, stamped with a head of a figure in relief,
which partially shielded the plate beneath from the rays.
The impression under the thinner portions of the medal-
lion is darker than under the thicker portions, thus
causing the head of the figure to be clearly apparent
in the photograph.
8 THE DISCOVERY OF RADIOACTIVITY
The Four Experimental Effects of Radio-
activity.
Although the radioactive process is itself without
analogy in science, the main effects which it pro-
duces can almost all be more or less nearly imitated,
and were all more or less perfectly studied prior to
its discovery. The main effects of radioactivity
with which we are most concerned are four. Firstly
then, radioactive substances affect a photographic
plate in the same manner as light and many other
agencies. Secondly, they excite phosphorescence or
fluorescence in certain substances when brought in their
neighbourhood. Thirdly, radioactive bodies cause the
air and other gases to lose the insulating power they
normally possess and to become partial conductors of
electricity. In consequence, any electrified object
has its electricity rapidly discharged in the neigh-
bourhood of a radioactive substance. The passage
of the rays through the gas shatters the electrically
neutral gas molecules into oppositely charged particles
or, as it is termed, ionises the gas. But the same
effect is produced by X-rays, by incandescent bodies,
and even by a lighted match. The instrument em-
ployed to detect this effect is the gold-leaf electro-
scope, the first and simplest electrical instrument to
be invented, and for this purpose capable of so great
refinement that it affords the most delicate and sensi-
tive test it is possible to employ in the detection of
radioactivity. Lastly, radioactive bodies generate heat,
as does coal or any other substance burning. The
photographic action and the discharge of electricity
from insulated charged bodies are clearly shown by
radioactive substances even in the form in which they
occur in Nature, as all unsuspected they have been
handled and examined by men for centuries. Hence
you will understand how it is that the discovery of
radioactivity could not under any circumstances have
EFFECTS OF RADIOACTIVITY 9
been indefinitely delayed. But only the more power-
fully radioactive substances, like radium, give appreci-
able phosphorescence or heat effects. In the naturally
occurring radioactive substances these effects are far
too small to be readily detectable.
The Rays of Radioactive Substances.
Exact physical experiments have demonstrated that
all these effects of radioactivity owe their origin to the
fact that the radioactive substances emit " rays." These
rays are invisible to the unaided eye it is true. In this
they resemble Rontgen's X-rays. There are three
different types of rays given out by the radioactive
substances, which are known respectively as the a-, /3-,
and 7-rays. Each will require detailed future considera-
tion. But they all bear less resemblance to light than
to the recently discovered types of rays, of which the
X-rays of Rontgen are typical, produced when an
electric current is forced by powerful appliances to
traverse a nearly vacuous space, a path which it much
prefers not to take if it can avoid it.
The first effects of most new things are old. Motor-
cars and railways do the old work of horses. In commer-
cial life a really new effect is generally valueless until it
has ceased to be new, as many inventors know to their
cost. In scientific discovery a new effect does not
usually proclaim itself from the housetops. It often
needs new instruments and the way must first be paved
for its discovery, while old effects are generally recog-
nised first. It is natural that the first effects of radio-
activity to be discovered should be those more or less
familiar. But for the development to perfection of
that marvellous thing, the photographic plate, radio-
activity would not have been discovered in the way it
was, and we should still be without one of the readiest
methods of detecting it. But for the work on the con-
duction of electricity through gases immediately follow-
ing the discovery of the X-rays, the only other method
10 THE DISCOVERY OF RADIOACTIVITY
of detecting radioactivity in the natural state would be
unknown, and therefore also in all probability radio-
activity itself. On the other hand, if radioactive
substances exhibit any entirely new kind of properties —
and it is quite possible that they do — it is very likely
that their very novelty would delay their discovery.
The Continuous Emission of Energy.
Why then, you may ask, if all of the effects of radio-
activity are shown in other ways do I insist that radio-
activity is a phenomenon unparalleled in science ? The
distinctive feature of radioactivity is not, however, so
much in the rays the radioactive substances emit,
though we shall find upon a closer examination that
these are distinctive and most remarkable. The main
interest of the new property consists in the spontaneous
and continuous emission of energy of which the rays are
but one manifestation. Heat and light may be obtained
in numerous ways, but it is a new thing to find it being
given out by a substance, as it is by radium, year in,
year out, without apparent intermission or diminution,
and without the substance being in any apparent
way consumed or altered. This was the arresting fact.
The radioactive substances apparently were perform-
ing the scientifically impossible feat of evolving a
store of energy presumably out of nothing. So long
as radioactivity was known only on the scale and
in the degree exhibited by uranium, it was perhaps
possible to explain away this aspect of the question
because of the minuteness of the amount of energy
involved and the difficulty of proving that it was not
in some way derived from the surroundings. But the
work of M. and Mme. Curie, by their discovery of radium,
made the world familiar with an element over a million
times as radioactive as uranium. In this case the
energy evolved is great enough to produce effects
which are obvious to all and which cannot be ex-
plained away. In a strictly scientific sense there is no
RADIOACTIVITY UNIQUE 11
difference of principle between the radioactivity of
radium and that of uranium. The difference is one of
degree only, but it is so great that radium, though, as
we shall come to see, not so wonderful in reality as
uranium, rapidly acquired a monopoly of public interest
and attention.
CHAPTER II
RADIUM
Radioactivity, an Unalterable Atomic
Property.
It is worth while to stop to consider the starting-point
of Mme. Curie's discovery. Chemistry analyses all
known substances into their component constituents
or elements, all of which are fundamentally different,
the one from the other, and inconvertible the one into
the other. Uranium is such an element, gold, silver
lead, and many of the common metals are others, but
uranium is distinguished by having relatively the
heaviest of all known atoms. The atom is the minimum
unit quantity of an element. The relative atomic
weight of an element is one of its most important charac-
teristics, and as a first approximation the atom of hydro-
gen is chosen as the standard and is assigned unit value.
For exact work it is more convenient to choose oxygen
as the standard, with the value 16. On this basis the
atomic weight of hydrogen becomes 1-008, and that of
uranium 238-18.
Now radioactivity is an intrinsic property of the
element uranium, and therefore of the atom of uranium.
This Mme. Curie first recognised, and it formed the
starting-point of her work. In the case of uranium,
the element itself and all its various compounds are
radioactive, and the radioactivity of each compound is
conditioned simply by the relative amount of uranium
it contains. It does not matter where the uranium
comes from — it is always to the same degree radio-
active. Non-radioactive uranium is unknown. Not
12
RADIOACTIVITY UNALTERABLE 13
only so, but it is absolutely impossible really to affect
the radioactivity of uranium or any other of the radio-
active elements in the slightest degree. In this the
process is utterly unlike any other process previously
known in Nature. Radioactivity is part and parcel of
the very nature of the element which possesses the pro-
perty, and therefore of the atom or unit quantity of the
element. The attempts that have been made artificially
to alter or to stop the radioactivity of an element have
met with signal failure. This is still an impossible feat
• — a thing modern science cannot do — and yet, as we
shall come to see quite clearly in the sequel, a thing
which science must do if mankind is to realise to the
full the destiny these discoveries have for the first time
unveiled. There is another still impossible feat, to the
accomplishment of which all the appliances of modern
science have been directed in vain, as well as all the
utmost power of man from the earliest time. It is
transmutation, or the conversion of one element into
another.
Radioactivity is the one process going on in matter
we cannot influence or stop, while transmutation is the
one process in matter we have so far signally failed to
effect. The juxtaposition of radioactivity and trans-
mutation is not a fanciful one, because it will appear,
as we proceed, that the two processes are most inti-
mately connected.
The Radioactivity of Thorium.
Radioactivity being a property of the element
uranium, it was natural to ask whether uranium alone
of all the eighty elements known possessed it. This was
the starting-point of Mme. Curie's illustrious researches
in the subject. She found only one other element
among those known which possessed the property — the
element thorium, which, at one time rare and little
known, has come into mdustrial prominence of recent
years in the manufacture of the Welsbach incandescent
14 RADIUM
gas mantle,^ of which it forms the main constituent.
To the electrical or ionisation test — the power of dis-
charging a gold-leaf electroscope — thorium prepara-
tions are of about the same degree of radioactivity as
uranium; but to the photographic plate thorium is far
less active than uranium, owing to the fact that the type
of rays which affect the photographic plate most strongly
are not those with most effect on the electroscope.
The radioactivity of thorium is a fact which can be
beautifully demonstrated by any one acquainted with
the process of photography. An incandescent mantle,
after burning off the fibre, is cut open and pressed as flat
as possible on a card. A photographic plate, which has
first been wrapped in a light-tight envelope, is laid upon
the flat mantle, and the whole is left undisturbed for a
fortnight or longer. On developing the plate it will be
found that an image of the mantle has been formed on the
plate in the dark by the rays from the thorium contained
in the mantle. Any one can do this simple experiment
for himself.
Fig. 2 (facing p. 7) shows the result I obtained with
a very thin piece of aluminium foil between the film and
the mantle. The foil, while quite opaque, allows the
a- as well as the /S-rays to go through. Paper would
stop the a-rays entirely.
The radioactivity of thorium, though producing the
same general effects as that of uranium, differs from
it entirely in detail. Indeed, by a few simple tests
on the radioactivity, any one of the radioactive
elements can be recognised and distinguished far
more quickly and certainly than by any of the other
chemical or spectroscopic tests, even when present
in very minute quantities. In the historical develop-
ment of the views now held in radioactivity thorium
played a leading part. But, as it is quite foreign to
my intention to give anything approaching a detailed
systematic account of the subject, and as radium lends
1 The cause of the action of the gas-mantle in generating light is
quite unconnected with the property of radioactivity.
Fig. 3. — Sir William Crookes' Pictures of Pitchblende
The lower figure is a daylight photograph.
The upper was imprinted in the dark by the rays from the substance.
To face p. 15
PITCHBLENDE 15
itself more readily to experimental demonstrations, I
shall confine myself at first to the properties of the
latter substance.
Pitchblende.
Although uranium and thorium were the only two
known elements possessing radioactivity, Mme. Curie
found that the natural minerals containing uranium
are more radioactive than can be accounted for by the
uranium present. Certain minerals, called pitchblende,
particularly the variety from the celebrated Joachims-
thal mine in Austria, contain often more than 50 per
cent, of uranium in the form of uranium oxide. The
radioactivity of pitchblende to the photographic plate
is beautifully shown by two photographs of Sir W.
Crookes (Fig. 3). The lower figure shows the polished
face of a piece of pitchblende photographed in the
ordinary way by daylight. The upper figure was
taken by placing the polished face of the mineral on a
photographic film wrapped in light-tight paper. The
lighter portions of the figure indicate where the plate has
been acted on by the rays from the radioactive matter in
the pitchblende. Some pitchblendes are from three to
four times as radioactive as pure uranium oxide. This
could only be the case, Mme. Curie correctly argued, if
there existed in the minerals one or more unknown
elements more powerfully radioactive than uranium.
By the ordinary process of chemical analysis it is easy to
separate out the various constituent elements in pitch-
blende. There are a great number of elements in pitch-
blende, though most of them are present in very small
amount. A fact that will be found significant later is
that lead is always present in important quantity.
Mme. Curie found that of the elements so separated two
in particular, the bismuth and the barium, were strongly
radioactive. Now ordinary bismuth and barium are
not at all radioactive, and the radioactivity of these
elements, when separated from pitchblende, is really
due to the presence of two new elements in minute
16 RADIUM
amount mixed with them. The one associated with
bismuth was discovered first by Mme. Curie and
named Polonium, after her native country. Its con-
sideration is more profitably delayed till later. The
other, which was discovered very soon afterwards, is
associated with the barium, and is Radium.
Quantity of Radium in Pitchblende.
The exact quantity of radium in pitchblende and
other uranium minerals is a fact of considerable impor-
tance. There is one part of the element radium for
every three million two hundred thousand parts of the
element uranium in pitchblende. The pitchblende may
be of any degree of richness, from only a few per cent,
to over 50 per cent, of uranium. But of even the
richest pitchblendes between 100 and 200 tons would
be needed to produce an ounce of pure radium. The
compound usually sold, hydrated radium bromide, the
formula of which is written RaBr2-2H20, contains, if
pure, 54-33 per cent, of radium. But what it lacks in
quantity radium makes up for in quality — that is to say,
in radioactivity. It is like the myriad of roses we are
told go to make a single drop of the real attar, which
is almost priceless. The radium that is extracted is a
million times more radioactive than the mineral, and
several million times more than pure uranium itself.
Conversely, just as you can buy quite a large bottle of
rose-water for a small sum, so quantity is not the only
consideration to be taken into account in the buying of
radium preparations. A very small quantity of radium
is sufficient to confer on a large quantity of an inactive
salt many of its own peculiar properties. Particularly
is this the case with the property of glowing visibly in
the dark. Weak radium preparations, which contain
usually barium, shine by themselves in the dark more
strongly even than the pure radium salts, owing to a
phosphorescent action of the barium salts, although
they may hardly contain enough radium to affect an
X-ray screen through a piece of metal. If you mix n
THE INTERNATIONAL RADIUM STANDARD 17
very minute quantity of radium with a quantity of a
very highly phosphorescent body, Hke sulphide of zinc,^
it will shine in the dark so brilliantly that an in-
experienced person might well be deceived into believing
that it must contain a large quantity of radium. So
great has become the need that radium preparations
should be of definitely ascertainable quality that in
1910 an International Radium Standards Committee
was formed, with the result that there is now preserved
in Paris an International Radium Standard prepared
by Mme. Curie, and consisting of a tube containing
twenty-two milligrams of the most carefully purified
radium chloride. By comparison with this standard
secondary standards have been prepared and supplied
to the official testing institutions of the various countries,
and now there is as much definiteness about the milli-
gram of radium as there is about a pound of tea.
The Smallest Quantity of Radium Detectable.
It is an interesting digression to consider here the
smallest absolute quantity of radium which can be de-
tected and identified with certainty in the laboratory.
One fifty-millionth of a milligram, or one three- thousand-
millionth of a grain of radium is quite easy to recognise,
whilst with special care one- tenth of this amount could
probably be detected. Thi§ is far less than could be
detected in the case of any non-radioactive element by
any method known, not excluding even the spectro-
scope. If the half of a grain of pure radium bromide,
which is in this room to-night, were divided equally
among every human being at present alive in the world,
and one such portion were returned to us, it would
prove sufficient for detection and identification by means
of a gold-leaf electroscope with the greatest ease. With
half a grain of a pure radium compound the main effects
of radioactivity, which in the case of uranium or thorium
would either be too feeble to show or would require the
1 This mixture now finds extended application in " Radium Watche.s'
and the like, for painting the dial figures and tips of the hands.
18 RADIUM
use of inconveniently delicate instruments, can be shown
in a striking and convincing manner to you all in the
simplest possible way.
Experiments with Radium.
Of the small amount of radium bromide, which by a
labour of love certain chemists have succeeded in ex-
tracting from pitchblende, I am fortunate to possess
about a grain, or sixty-five milligrams. Half of this
quantity, which I shall use for most of my lecture
experiments, is contained in a small ebonite capsule.
The other half is dissolved in water and not brought
into this lecture-room, but kept in the laboratory half
a mile away. With the room dark the radium in the
capsule is hardly visible to you, because the rays do not
of themselves affect the unaided eye, but if I bring some
crystals of the fluorescent substance, barium platino-
cyanide, near to it, you will see that the crystals shine
out at once with a beautiful green light. An ordinary
X-rays fluorescent screen, which is simply a piece of card
painted over with the same fluorescent substance in the
form of powder, is very convenient for these experiments.
When thin pieces of metal foil are placed between
the radium and the crystals you see their brightness is
only slightly reduced, while several shillings can be in-
terposed one above the other without altogether stop-
ping the rays from the radium. Those in the front will
see the crystals still shining faintly, although the rays
from the radium have first to traverse more than half
an inch of solid silver before reaching the crystals. The
electrical effect of radioactivity can be shown in a very
rough and simple way with this, comparatively speaking,
large quantity of radium. A silk tassel is stroked with a
rubber tobacco-pouch and so electrified. All the threads
then repel one another and stand out as you see (Fig. 4).
The moment the radium is brought near the threads
collapse at once (Fig. 5). Lastly, the photographic action
of the rays is seeh in the photograph (Fig. 6, facing p. 81)
t)
Q
r/1
<
C
Pi
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S
W
O
y^j
Pi
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H
rn
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f4
To face p. i8
COST OF RADIUM 19
which was obtained by slowly writing, with a small
tube containing a small fraction of a grain of radium
bromide as if it were a pencil, over a photographic
plate wrapped in black paper, and then developing the
plate without exposure to light.
By the aid of delicate thermometers it could also be
shown that this small quantity of radium is always a
few degrees hotter than the surrounding air.
Cost of Radium.
The one fact about radium, which every one is aware
of, is its tremendous cost. When you consider that even
of the best ore several hundredweights must be worked
up to obtain the small quantity here exhibited, you
can understand that the price is necessarily very higii.
The price rose rapidly from about 8s. the milligram for
radium bromide in 1903 to about £15 the milligram in
1912, and even at the latter price very inferior prepara-
tions have found a ready sale. During the war, in
which radium found several applications for illuminating
rifle sights, compass cards and the like, over £20 a
milligram was paid, and it is likely to remain at this high
level. We shall see, as we proceed, that from its very
nature any strongly radioactive body like radium must
always be excessively rare. Indeed, in the degree of
radioactivity we have a scientific standard of rarity,
and therefore of " value." There are unfortunately
some fields of scientific investigation, of which radio-
activity is one, which cannot be thoroughly explored
without continuous and considerable expenditure. The
old boast of science, that some of her grandest dis-
coveries were made with very simple apparatus, largely
built up of wire and sealing-wax, costing little or
nothing, does not apply to any of the discoveries with
which we are now concerned. The investigations of
Mme. Curie naturally have cost many thousands of
pounds, provided in part by the Austrian Govern-
ment and the Rothschilds. This radium we are using
H
20 RADIUM
to-night we owe to the work of a German chemist,
Dr. Giesel, who undertook its extraction on a large
scale in the early days when the raw material was to be
obtained in the market, and who very unselfishly dis-
tributed much of the radium he prepared among workers
in all parts of the world.
The chief source of radium at the present time (1920)
is American carnotite, which contains 2 per cent, of
uranium in the form of a uranium potassium vanadate,
mixed with sandstone. Its use in medicine and, later,
in war has produced results as startling in the field of
common sense as in that of physical science. " A
great industry has sprung up." The creator of the
wealth, the scientific investigator who discovered the
material and the methods of winning it, and made a free
gift of all his hard-won knowledge to the community, is
now unable to afford to buy radioactive materials, even
in the modest quantities he needs for scientific investi-
gation.
The Doctrine of Energy.
To-night it is not my intention to take you through
the various phases of the new properties of radium.
We have to face squarely the great general question
which its simple existence has demanded of physical
science. Last century will remain for ever memorable
on account of the development and establishment of
the great doctrine of energy. Those were splendid days
for physical science in Scotland, for that doctrine, which
lies at the root of all modern industry and enterprise,
took its rise largely in Scotland, and was developed by
Tait, of Edinburgh, and Lord Kelvin, of Glasgow.
For a full account of these stirring developments you
should read Tait's Recent Advances in Physical Science,
which, in spite of the fact that it is now over forty years
old, still continues fresh and inspiring. The first law,
that of the conservation of energy, states that energy is
a real entity, and has a real existence no less than matter,
and no more than matter can energy be created or de-
THE DOCTRINE OF ENERGY 21
stroyed, although the forms it may assume are legion.
The second law, that of the availability of energy, is
sufficiently accurately stated for present purposes by
saying that the same energy is available for useful work
but once. To obtain useful work from any source of
stored-up or potential energy, it is necessary to trans-
form it into new forms which are kinetic, and by which
something is made to move. As motion is invariably
attended by friction or similar processes, ultimately the
energy passes into heat. It is said to be degraded into
low-grade or waste energy, for although all forms of
energy tend, after assuming the kinetic form, to turn
into heat, the transformation of the waste heat so pro-
duced back into useful forms cannot be practically
effected. The conversion is not altogether impossible,
but requires for its accomplishment the degradation of
more fresh energy than is gained, and so is practically
out of the question.
The practical aspect of the question may be summed
up by saying that if you want useful energy, you must
pay for it like any other commodity, and the value of the
energy, though not the energy itself, is destroyed by use.
The up-to-date street car driven by the electric motor,
which has displaced the old horse-tram, although it has
not the same obvious incentive to locomotion as its pre-
decessor, nevertheless does not go by itself. It requires
energy or power, which is bought and sold and has a
value as strictly as the oats and hay which energised the
now emancipated horse. The driving power of the
machinery of the modem world is often mysterious, but
the laws of energy state that nothing goes by itself, and
our experience, in spite of all the perpetual motion
machines which inventors have claimed to have con-
structed, bore this doctrine out, until we came face to
face with radium. Nothing goes by itself in Nature,
except apparently radium and the radioactive sub-
stances. That is why, in radioactivity, science has
broken fundamentally new ground.
I cannot too plainly insist that available energy,
H
22 RADIUM
though immaterial and intangible, has a definite and
real physical existence. Were it not so, coal would not
be the very expensive commodity it unfortunately is
rapidly becoming. No one burns coal for the sake of
polluting the atmosphere, but simply and solely because
it gives out during combustion a certain amount of
energy as light or heat. Last century civilisation may
be said to have attained its majority and to have entered
upon the control of an inheritance of energy stored up
by the sun in fuel during the long ages of the past, and
now it is dissipating that inheritance as quickly as it can.
With the light -heartedness and irresponsibility of youth,
it is taking no thought of the future, but confidently
assumes that the supply of natural energy, upon which
at every turn it is now entirely dependent, will continue
indefinitely. Well ! if it does not do so, new stores of
energy cannot be created to order, and there will be an
end to the age of energy in which we are living, and to
civilisation as we have come to understand it.
Measurement of the Energy emitted by
Radium.
Energy is susceptible of exact measurement and,
though it exists in many varieties, all forms of energy
can be most readily and completely converted into heat
and measured as such. The energy given out by radium,
although it is in nature new, is no exception to this rule.
Practically the whole of the energy is transformed into
heat when the radium is kept in a leaden vessel, so that
the rays are absorbed in the surrounding metal. The
actual amount of heat given out, for instance, by this
small quantity on the table is, of course, very small, but,
in comparison with the quantity of substance producing
it, it is very great indeed. Exact experiments have
proved that 1 gram (=15-4 grains) of radium gives out
133 calories per hour.^ The amount of heat evolved by
1 The calorie is the quantity of heat required to raise 1 gram of
water 1° Centigrade. Spelt with a capital C, the Calorie is 1,000 calories
HEAT EVOLVED BY RADIUM 23
any quantity of radium in three-quarters of an hour is as
much as is required to raise a quantity of water equal
in weight to the radium from the freezing-point to the
boihng-point. Radium bromide, if it is dry, consists
roughly of three-fifths by weight of the element radium
and two-fifths of the element bromine. Half a grain of
radium bromide thus evolves two and a half calories
every hour. This specimen of half a grain of radium
bromide has been in my possession for sixteen years,
and the outpouring of energy has been going on cease-
lessly day and night at a steady rate. A simple calcula-
tion shows that in this time about 350,000 calories have
been evolved. To obtain an idea of what this means
consider the amount of energy given out in the burning
of coal. A weight of coal equal to the weight of this
radium bromide would give out during complete com-
bustion only about 250 calories, so that this radium has
evolved in sixteen years 1,400 times the energy obtain-
able from the same weight of coal. I have chosen coal
for the comparison because the combustion of carbon
furnishes the modern world with its main supply of
energy. During the last sixteen years this radium
has given fourteen hundred times as much energy as
could be obtained from an equal weight of any other
kind of substance in any way known. Coal is no longer
coal when it is burnt and consumed. Gunpowder and
dynamite, once they have exploded and evolved their
stored-up energy, disappear as such, and there remain
incombustible and non-explosive solids and gases, out
of which no more energy can be drawn. But this radium
is as active as ever. So far, careful measurements have
failed to detect the least diminution in the radio-
activity of radium with time. Rather it increases
steadily, rapidly in the first month and slowly for the first
few years after preparation, for certain profound reasons
we shall have to go into subsequently. These show also
that after some thousands of years the evolution of
energy must cease. But the calculated diminution is
only some four per cent, per century.
24 RADIUM
The Source of Cosmical Energy.
In the face of a new fact of this character it is obvious
that this doctrine of energy, which we thought so well
founded, requires further consideration. Based as it
has always been on the results of our experience and the
practical impossibility of achieving perpetual motion of
any kind, it is confronted with a natural example,
going on apparently for an unlimited space of time
under our very eyes, which not only does not come to
a stop, but which cannot be stopped by any means what-
ever. Now, although the doctrine of energy accords
well enough with our terrestrial experiences, the student
of the physical sciences has only to turn his thoughts
from the laboratory to the heavens to see there, in the
larger laboratory of Nature, an example of practical
perpetual motion on the grandest and most majestic
scale. What, for example, is the source of the apparently
inexhaustible supply of energy from the sun, upon the
receipt of a minute and insignificant fraction of which
life on this planet absolutely depends for its continued
existence from year to year ? This is a question which
has been frequently asked and only imperfectly answered
by physical science. It has been the custom vaguely to
connect the apparently endless and inexhaustible out-
pourings of energy going on everywhere in the universe
with its vast scale and dimensions. In the background
there has always been the tacit assumption that the
supply of fresh energy is only apparently inexhaustible,
and that in some remote future a time will at length
arrive when the supplies of fresh energy are exhausted
and all things will come to a stop and remain at rest for
ever. We have applied the teachings of the labora-
tory, our knowledge of the laws of energy and its con-
servation, and the impossibility of perpetual motion,
without modification to the cosmos, only making allow-
ance for its enormous scale.
Astronomers, in consequence of the new discoveries,
QUOTATIONS FROM PROFESSOR TAIT 25
are no longer compelled to regard cosmical evolution as
proceeding on these old conventional lines. It is not so
certain as it was that it is only a question of time before
the sun and planets cool down to a dead uniform tem-
perature. In former days this point of view was the
only possible one. A hot body radiating heat and light
into space, even when all possible sources of energy, such
as the accretion of meteorites, shrinkage, etc., have been
allowed for, must ultimately radiate away its energy.
The same is still true but with a difference. Thus
Professor Tait, in his Recent Advances in Physical
Science (1876), says (p, 169): " If we were to trace the
state of affairs back, instead of to ten millions, to a
hundred millions of years, we should find that (if the
earth then existed at all) if that collocation of matter
which we call the earth was then actually formed, and
if the physical laws which at present hold have been
in operation during that hundred million years, then
the surface of the earth would undoubtedly have been
liquid and at a high white heat, so that it would have
been utterly incompatible with the existence of life of
any kind such as we can conceive from what we are
acquainted with. Thus we can say at once to geolo-
gists, that granting this premiss — that physical laws
have remained as they now are, and that we know of all
the physical laws which have been operating during that
time — we cannot give more scope for their speculations
than about ten or (say at most) fifteen millions of years.
" But I dare say many of you are acquainted with the
speculations of Lyell and others, especially of Darwin,
who tell us that even for a comparatively brief portion
of recent geological history three hundred millions of
years will not suffice.
" We say, so much the worse for geology as at present
understood by its chief authorities, for, as you will
presently see, physical considerations from various inde-
pendent points of view render it utterly impossible that
more than ten or fifteen millions of years can be granted."
Again (p. 154): "Take (in mass equal to the sun's
26 - RADIUM
mass) the most energetic chemicals known to us, and in
proper proportion for giving the greatest amount of
heat by actual chemical combination, and, so far as we
yet know their properties, we cannot see the means of
supplying the sun's present waste for even 5,000 years.
. . . This question is totally unanswerable, unless there
be chemical agencies at work in the sun of a far more
powerful order than anything that we meet with on the
earth's surface."
Radium and the " Physically Impossible."
I do not quote these utterances with any wish to
revive the old controversy between geologists and
physicists, long since tacitly abandoned by both sides
mutually as barren and unprofitable, but because of their
present extraordinary aptness. To-day, science has
come to know, by means of radioactivity, of agencies at
work on the earth's surface of a far more powerful order
than anything that was known in the time of Professor
Tait. The discovery of radioactivity and the revela-
tion it has given of unsuspected stores of energy in
Nature available for cosmical purposes, of necessity put
the whole question of the evolution, the past history and
the future destiny of the universe in a new light. This is
one of the conclusions of clearly general interest which
follow from the recent discoveries.
There is nothing of the vast scale and dimensions of
the universe about this tiny scrap of radium. Yet it is
giving out energy at a rate, relative to its mass, which no
sun or star is doing. Suppose, for example, our sun,
instead of being composed of the materials it is, which
we know by the spectroscope are practically the same
as those of the earth, were made of pure radium. Pro-
vided only that every part of its mass gave out energy
at the rate this radium on the table is doing, there would
then be no difficulty in accounting for its outpourings of
energy. Rather, the light and heat that would be given
out from such a sun would be of the order of a million
times greater than they actually are. On another count
RADIUM AND THE STARS 27
also one's thoughts almost unconsciously revert from
radium to the transcendental phenomena of the larger
universe, for in no other phenomena are we so reduced
to the position of onlookers, powerless alike to influence
or control. All the powerful resources of the modern
laboratory — extremes of heat and cold, and of pressure,
violent chemical reagents, the action of powerful ex-
plosives and the most intense electrical agencies — do not
affect the radioactivity of radium or the rate at which
it works in the slightest degree. It draws its supplies
of energy from an hitherto unknown source and obeys
as yet undiscovered laws. There is something sublime
about its aloofness from and its indifference to its exter-
nal environment. It seems to claim lineage with the
worlds beyond us, fed with the same inexhaustible fires,
urged by the same uncontrollable mechanism which keeps
the great suns alight in the heavens over endless periods
of time. This tiny speck of matter we can hold in
our hands exhibits in perfect miniature many ancient
mysteries, forgotten almost in their familiarity, or mis-
takenly and too easily dismissed as belonging and
appropriate to the infinitely great dimensions of the
universe. The " physical impossibility " of one era
becomes the commonplace of the next, and in the con-
troversy between the geologists and the physicists we
have a good illustration that no theory can claim a
universal application. It is of necessity partial, and
bounded on all sides by the unknown and unexplored.
It is rarely proved false, so surely and truly are the
foundations of modern science laid, but it is liable
at any moment to be restricted in its application to
the particular cases for which it was formulated and
found not to apply in new spheres at the time of its incep-
tion unsuspected. As we shall see, the law of the con-
servation of energy is not necessarily controverted by
any of the new facts with reference to radium, but prior
to these discoveries our knowledge of the available
sources of energy in Nature has been partial and super-
ficial to a degree.
CHAPTER III
RAYS OF RADIOACTIVE SUBSTANCES
The Radiations of the Radio-Elements.
In the previous lectures we have considered the bare
fact that radium and the radioactive substances are
continually evolving from themselves a perennial
supply of energy, and the fundamentally new ground
which this discovery opens up in physical science.
To-night our inquiries will be directed to one special
portion of the subject, namely, the nature of the rays
emitted by the radioactive elements, by means of which,
or rather of the effects of which, the property was first
discovered. These rays themselves, apart from their
effects, we have hitherto scarcely considered, but they
play an essential part in the theoretical scheme by which
the activity of the radio-elements is now interpreted.
The tracing back of the main effects of radioactivity,
photographic, fluorescent, electrical, and thermal, to
definite radiations emitted by the radio-elements came
very early in the subject, but it must not be for-
gotten that such tracing back is of the essence of
the discovery. Too frequently it is wrongly assumed
without such evidence that any substance capable of
simulating one or other of the various effects of radio-
activity is therefore a radioactive substance. Natur-
ally, the exact study of the new radiations has been
mainly the work of physicists. They have succeeded,
not only in clearly analysing into distinct classes the com-
plex radiations involved and distinguishing the part
played by each alone, but also they have advanced
very far towards a solution of the real nature of each
28
a-, y8- AND r-RAYS 29
class of radiation emitted. Much of this latter work,
however, is based upon reasoning of too specialised and
intricate character for general presentation, and as these
lectures are intended primarily for the general public, and
not for trained physicists, I propose concentrating
attention for the most part on the conclusions which
are universally accepted and of the greatest general
interest. Although the reasoning is difficult, the chief
conclusions are very simple and easily followed, and
they fit in with the general scheme of the cause and
nature of radioactivity in a way which makes the whole
subject clearer and more easily visualised.
a-, /3- AND 7-Rays.
The first analysis of the complex radiations emitted
by each of the radio-elements — uranium, thorium, and
radium — was done by Sir Ernest Rutherford, and much
of the work we are considering is his, and has called
forth in their highest degree his well-known experi-
mental genius and energy. He classed the rays
into three main types, the a-, ^- and 7-, distinguished
from one another by enormous differences in their power
of penetrating matter. I may say at once that the a-
rays of radium, for instance, are readily distinguishable
in penetrating power from the «-rays of uranium, and
the latter again from those of thorium. Moreover,
the a-rays of radium are themselves complex and con-
sist of no less than four separate types readily dis-
tinguished. The same is true of the yS- and 7-rays of
radium, which are themselves complex and recognisably
different from the /3- and 7-rays of uranium or thorium.
But the differences between the a-rays as a class, for ex-
ample, are small and unimportant relatively compared to
the enormous difference between any a-ray and any
/S-ray or 7-ray. The most penetrating a-ray known is
not much more than twice as penetrating as the least
penetrating known, whereas the /3-rays as a class may
be considered to be approximately a hundred times more
4
so RAYS OF RADIOACTIVE SUBSTANCES
penetrating than the a-, and the 7-rays a hundred times
more penetrating than the /S. Again, the kind of
matter penetrated, although it has a certain influence
which may be different for different types of rays, is
only of secondary importance. For these rays, like
the new X-rays, and unlike light, are absorbed by matter
roughly in proportion to its density, and quite indepen-
dently of its optical qualities of transparency and opacity.
The first result of these researches was to bring into
prominence the o;-class of rays, which at first sight are of
apparently little importance, and to diminish relatively
the importance of the /3-class of rays which had been
operative in the photographic effects hitherto mainly
studied. For the a-rays are completely absorbed by
very thin screens — even by a sheet of thin paper, or by
three inches of ordinary gaseous air, — and they produce
but little action on the photographic plate in com-
parison with the ^-rays, which are able to pass through a
visiting card or piece of tinfoil with ease. To the electri-
cal test — the discharge, for example, of an electrified
silk tassel or electroscope — ^the a-rays are immensely
more effective than the /3- and 7-rays together, and from
this fact Rutherford concluded, and the conclusion has
been wholly borne out by subsequent developments,
that the energy possessed by these feebly penetrating,
and not at first sight very striking, a-rays is always
immensely greater than that of the other two types
taken together. In fact, the yS- and 7-rays at most
possess but a few per cent, of the total energy of radia-
tion, and therefore are in this fundamental respect rela-
tively of less consequence than the previously neglected
a-class. Although less suited to lecture experiments
than the other more penetrating types, the a-class have
proved far the most instructive and important in the
theory of radioactive change.
Fig. 6. — Written by Radium in the Dark.
(From a Radiograph by R. Hill Crombie, Esq., Jou7-nalofthe Rontgeii Society, Dec, 1906.)
I'lG. 7. — Closed Bo.x: of Co.mpasses taken with the 7-Rays of Radium.
To face p. 31
THE PENETRATING RAYS 31
Experiments with the Penetrating /3- and
7 Rays.
The small capsule in which my radium is contained
is closed by a thin sheet of mica, which effectively stops
all the a-rays, so that in working with the capsule only
the yS- and 7-rays are operative. The platinocyanide
salts fluoresce most brilliantly under the /3-rays. On
interposing successive thicknesses of thin copper or
aluminium foil the fluorescence is weakened, very rapidly
at first, but a point is soon reached when the feeble
fluorescence remaining is not much further weakened
by additional thicknesses of foil. This is because the
/3-rays have all been absorbed, and there remain only
the relatively feeble but extraordinarily penetrating
7-rays. These 7 rays are always very feeble, and com-
paratively unimportant, but their chief interest lies in
^he fact that they are by far the most penetrating type
__of radiation at present known. If the capsule is com-
pletely closed in a box of steel, half an inch thick, and a
platinocyanide crystal laid on the top, those in front can
readily see that the crystal still fluoresces, and stops the
moment it is taken away from the radium. Through a
pile of twelve shillings, or pennies, the effect can still be
observed, while by means of a sensitive gold-leaf elec-
troscope it has been shown that a minute proportion of
the rays can penetrate a foot thickness of solid lead.
The rays from radium are not well adapted for the
taking of radiographs of the kind produced by X-rays.
The )S-rays are hardly sufficiently penetrating for this
purpose, so that the flesh as well as the bones of the hand,
for example, casts a heavy shadow. The 7-rays, on the
other hand, are far too penetrating, and *^he bones hardly
cast a shadow at all. The picture (Fig. 7), however,
is a good example of a radium radiograph taken by the
7-rays of radium. A small box of compasses with the
lid shut was placed on a table. Over it, film down, was
placed an X-ray plate wrapped in a light-tight envelope.
\
82 RAYS OF RADIOACTIVE SUBSTANCES
On the floor beneath, at a distance of twenty-five inches
from the plate, was placed one-tenth of a grain of pure
radium bromide sealed up in a tiny glass tube. The
radium was placed between the poles of an electro-
magnet, as recommended by Mme. Curie, to deflect
away the /S-rays which tend to blur the distinctness of
the picture. "IntHis way the 7-rays of radium were
alone used. The exposure was five days. It will be
seen that the shadow cast by the wooden box is scarcely
noticeable, while even the metal compasses and fasten-
ings of the box by no means entirely stop the rays.
The metal parts appear in the negative only slightly
darker than the unprotected portions of the plate. The
negative was reduced and intensified before repro-
duction.
At first the 7-rays appeared to be a secondary radiation
produced by and accompanying the yS-rays, much as
X-rays are produced by and accompany cathode-rays.
The /3- and 7-rays seemed always to go together, any
variation of the )S-rays being accompanied by a
similar variation of the 7-rays. This is now known,
however, not to be invariably the case, and the opinion
is gaining ground that the ^- and 7-rays are not
necessarily connected. The question of the real nature
of the 7-rays was the last to be solved, and as the rays
are not of primary importance at the present stage we
may, with these experiments and remarks, defer the
subject and pass on to the more detailed consideration
of the two more important types of rays.
The Feebly Penetrating k-Rays.
Before proceeding to show experiments with the a-rays
it is necessary to touch on certain considerations which
come into play on account of their very great absorp-
tion in passing through matter. In the first place,
radioactivity is a mass or volume phenomenon. That
is to say, every part, not the surface only but the inner
portions also, of a radium salt, for example, is giving
THE FEEBLY PENETRATING RAYS 33
out a-, y8- and 7-rays. All these rays are absorbed
by the substance itself very considerably, for the salts
of radium are dense or heavy. But this absorption
naturally does not affect the more penetrating rays
nearly so much as the feebly penetrating a-rays. That
part of the latter, generated inside the salt, does not
escape at all. Only a very thin surface film contributes
to the a-radiation. The consequence is that whereas,
with the small quantities of radium that we have to
work with, the strength of the penetrating rays is more
or less proportional to the quantity of radium employed,
with the a-rays this is no longer the case. The weight
of the substance is less important than the amount of
surface exposed. A very small quantity, say a milli-
gram, of radium bromide, spread out as a thin film on a
large plate, will give out immensely more a-rays than
the same quantity in the form of a small crystal. In
order to free the ^- and 7-rays from the a-rays, or the
7-rays from the ;S-rays, it suffices to interpose screens of
successively increasing thickness until the more easily
stopped type is completely absorbed. But it is not
possible so easily to eliminate by physical methods the
/S- and 7-rays from the a-rays in order to leave the latter
by themselves. For practical purposes, however, this
result can be achieved very simply. If we take a very
minute quantity of radium salt spread over a very large
area, the ^- and 7-rays from so small a quantity will be
so feeble as to be practically negligible, whereas the
a-rays under these circumstances will reach their
greatest intensity. For practical purposes a thin film
of pure radium salt can be used to give a-rays by
themselves, essentially free from yS- and 7-rays.
Experiments with c-Rays.
Such a thin film I have prepared for these experi-
ments. On this shallow platinum dish, about a square
inch in area, I have evaporated down a solution contain-
ing about a milligram of pure radium bromide, and the
34 RAYS OF RADIOACTIVE SUBSTANCES
dish, with its precious film open to the air, is carefully
preserved when not in use in a special tube containing
a desiccating agent to keep it dry, so that without undue
risk of loss I can work with a bare film of radium salt
and show you the a-rays. Over the bare film I bring
the electrified silk tassel. It collapses instantly, in fact,
much faster than it does when brought over the whole
thirty milligrams of radium bromide contained in the
mica-covered capsule. The a-rays from one milligram of
radium produce more electrical effect than the 13- and
7-rays from thirty milligrams. Now I cover the bare
film of radium with a single sheet of thin writing-paper,
which stops the a-rays completely, the /S- and 7-rays
scarcely at all. You observe the tassel remains now
charged as if the radium were absent. The /3- and
7-rays from so small a quantity hardly appreciably
discharge it.
But if I displace the paper ever so slightly and
expose a tiny part of the bare surface, the tassel instantly
collapses. From these experiments, and the fact that
it was the fashion at the time to cover radioactive sub-
stances when experimenting with them, you will have
no difficulty in understanding how it was that these
feebly penetrating but intensely powerful a-rays re-
mained at first neglected and almost unknown.
The Range of c-Rays in Air.
I now have to show you a very striking experiment
indeed, suggested by some profound investigations of
Professor Bragg in Adelaide, on the a-rays, to which we
shall again have occasion to refer. So readily are these
a-rays stopped that a few inches of air suffice entirely
to absorb them. But the a-rays show this remarkable
peculiarity not exhibited by any other type known.
Each individual a-ray of any one homogeneous type
travels exactly the same distance in an absorbing
medium, and is stopped sharply and completely when a
certain thickness of matter has been penetrated. The
Fig. 9. — ArrARATUs to show Absorption of o-Rays by Air.
Tig. II.— The SriNTHARiscoi'E of Sir William Crookes.
To face p. 35
RANGE OF a-RAYS
35
consequence is that if we work with a homogeneous
beam of a-rays, just without the distance of complete
absorption, there is absolutely no effect, while just
within there is a very large effect. I have said that the
a-rays derived from radium are complex, consisting of
four different types, each with a definite " range," as it
is termed, or distance, it will travel in any given absorb-
ing medium. For the purposes of this experiment,
however, it is necessary to consider only the most
H
Fig. 8.
— ^PUMP
penetrating type, which Bragg found could travel in
air at atmospheric pressure and ordinary temperature,
71 millimetres (or just under three inches) and no
more. Now this flask (Figs. 8 and 9) is a little more
than six inches in diameter, and it has been coated
on the upper hemisphere of the inside surface with a
phosphorescent film of zinc sulphide. For these a-rays
the usual phosphorescers [e.g., the platinocyanides,
willemite, etc.), employed for the /3- and 7-rays, are
far less sensitive than crystallised zinc sulphide, or, as it
y
36 RAYS OF RADIOACTIVE SUBSTANCES
is called, Sidot's hexagonal blende. The coated flask
is arranged so that I can plunge my platinum dish with
its bare radium film upward inside the flask and hold it
centrally by a cork. In the dark, the flask being full
of air, you observe hardly any glow. The three inches
of air surrounding the radium film on all sides suffices
completely to stop all the a-rays, and the /8- and 7-rays,
from so small a quantity of radium, produce only a
negligible effect on the zinc sulphide. But I have con-
nected the flask to an air-pump and can pump out the
air. At the very first stroke of the pump the whole
globe flashes into luminescence, and as I continue
pumping the glow gets stronger and fairly illuminates the
immediate neighbourhood with its soft white light. I
now readmit the air, and the glow disappears as suddenly
as it came. So that you see, with somewhat carefully
designed arrangements, and keeping in mind the peculiar
properties of these a-rays which physicists have exactly
worked out, it is possible even from a minute amount
of pure radium bromide to obtain quite a fair amount
of light, whereas the same quantity of radium less
cunningly disposed would give very little effect. Radium
compounds are usually preserved in sealed tubes so as
to prevent them absorbing moisture from the atmo-
sphere. Under these circumstances the effects produced
by these a-rays are not observed.
The Physical Nature of Radiation.
Problems connected with the real physical nature of
radiation are, it is well recognised, among the most
fundamental in physics, and they involve more deeply
perhaps than any others the great underlying meta-
physical relationships between the external world of
physical fact and the subjective mental processes by
which we attempt to visualise these facts and obtain
some sort of a reasonable explanation of them. Take,
for example, the great problem that is always before us
of the real nature of light. Is there anything more
RADIATION 37
difficult of mental comprehension ? The difficulties
are not minimised but rather increased by the very
definite view we take to-day of energy as a separate
entity having a real physical existence.
Contemplate for a moment, if you can, the origin of
the energy which impels every moving thing in earth or
sea or sky. With the exception of a very small and
practically negligible movement contributed by the
tides and by volcanic agencies, and, it must not be for-
gotten, by the radioactive substances themselves, all
things which move do so directly or indirectly by virtue
of the energy reaching this earth as radiations in the
form of the sun's light and heat. Great masses move
hither and thither here because of happenings at some
time past, remote or recent, 90 millions of miles away in
the sun. Inevitably, when we begin to contemplate
radiation phenomena, we are driven to inquire into the
medium filling the outer void of space by virtue of which
this immaterial, but vital entity — energy — reaches us
from far distant worlds. It is true we call it ether,
and try to give to it all sorts of material, or pseudo-
material, characteristics. Lord Kelvin seems to have
spent a large part of his leisure time trying as it were to
dematerialise matter into ether, that is, trying by all
sorts of mechanically ingenious arrangements and
analogy from material models — the only possible models
our minds can yet grasp — to obtain a possible con-
struction which would simulate the elusive but all per-
vading ether. Others, on the well-known principle that
topsy-turvydom, if only consistent and all-embracing
enough, results finally in a system no less logical and
rational than the original one, have given to the ether
inconceivably great density, and to the atoms of matter
the character of holes or voids in it. The necessity for
the existence of a universal all-pervading medium, or
ether, capable of transmitting energy, no one in these
days of wireless telegraphy would deny, but on the
question of its real nature opinion is as divided as it well
could be.
88 RAYS OF RADIOACTIVE SUBSTANCES
The tendency, however, in modern physics to-day is
rather to derive and explain material phenomena from
the properties of the ether than to attempt to construct
an ether on a material or pseudo-material model. As
yet, however, we know little about the properties of the
ether itself. One definite thing we do know, for certain,
and have known for a very long time, namely the
velocity at which influences are transmitted across the
ether. It is 185,000 miles a second, the speed of light.
So far as we yet know, all influences that are transmitted
by the ether travel at this one definite velocity. Not
only light, but also the electro-magnetic radiations
employed in wireless telegraphy, the magnetic storms,
as they are termed, which reach us from the sun, and
also, we believe, the X-rays, travel through the ether
at this one definite speed.
Corpuscular Radiation.
The great mind of Newton two centuries ago ap-
preciated to the full the fundamental difficulty in the
explanation of radiation, and proposed the only way of
escape from the more modern doctrine of an ether which,
so far as I know, has ever been put forward.
Light, on the Newtonian hypothesis, consisted in the
emission from the glowing body of excessively minute
material particles or corpuscles travelling with immense
velocity. This corpuscular theory, so far as light is
concerned, failed when subjected to a closer examination,
and gave way to the present undulatory theory that light
consists in a transverse vibration of the ether, the exist-
ence of which, it was beginning to be recognised, was
as great a necessity for the transmission of gravitational,
magnetic, and other forms of energy which reach us
from outer space as it was for the transmission of radia-
tion itself. Though proved wrong so far as light is con-
cerned, this idea of corpuscular radiation, strangely
enough, will rank as one of the most suggestive flashes of
Newton's genius, for it, in fact, anticipated by two
CORPUSCULAR AND WAVE-RADIATION 39
centuries the march of experimental discovery. To-day,
thanks to radioactivity, science has been enriched by the
discovery of a-, y8-, and 7-rays, and two, at least, out of
these types, the a- and the yS-rays, are not, like light,
vibrations of the ether, but consist of the emission of
excessively minute material particles (atoms and cor-
puscles) travelling with immense velocity. This is one
of two chief main lines of evidence that radioactivity is
an accompanying manifestation of " atomic disinte-
gration."
Into this aspect of the matter, however, I do not pro-
pose entering to-night. Its consideration is more con-
veniently deferred. It is sufficient to say that the a- and
yS-rays, or, as I shall henceforth also refer to them, a-
and y8-particles, comprise, the lighter fragments, as it
were, of the disintegrating atoms of the radioactive sub-
stance. In ordinary circumstances radium appears to
be expelling both a- and /^-particles together, but this
as we shall come to see is due to the fact that several
successive disintegrations are occurring, and the effect is
a composite one. The nature of these rays is so utterly
different from that of light that it is worth while to stop
and examine the difference a little more closely.
The Wave Theory of Light.
The wave theory of light has often been illustrated
by what happens when a stone is dropped into a pool.
Ripples extend outwards in concentric circles from the
disturbance. The water, as the ripple reaches it, first
rises above, then immediately afterwards falls below
the normal level. The disturbance is propagated trans-
versely, that is, outwards horizontally by a vertical,
or up and down wave-movement of the water. The
surface discloses the nature of the disturbance, but the
same type of disturbance is taking place below the
surface, and each circular ripple is in reality the section
of a hemispherical shell. It is not possible to get an ether
surface like a water surface, since the ether is all-per-
40 RAYS OF RADIOACTIVE SUBSTANCES
vading. Light travels out from an incandescent point
in all directions in spherical ripples, in which a to-and-
fro motion of some kind is going on in the ether, trans-
verse to the direction of propagation of the light. Con-
trast with this what is believed to be the nature of the
a- and /3-rays given out from a radioactive substance.
The rays are given out uniformly in all directions, not as
a succession of spherical waves, but as the random flight
of immense swarms of tiny projectiles ejected from the
radioactive substance. For shortness I shall call this
the " discrete theory," as contrasted with the wave
theory, because the radiation is considered to be due to
the flight, radially outward from the substances like the
spokes of a wheel, of swarms of free-flying, independent
discrete particles. You could hardly imagine two more
different phenomena, and yet that it is not easy to dis-
tinguish between their effects is shown by the fact that
for a long time a controversy raged between the two
views regarding the nature of light itself.
a- and is-rays due to the expulsion of
Particles.
I must anticipate a little here for the sake of clearness.
It is now an old story that in the tiniest grain of matter
there is a mentally inconceivable myriad of separate
atoms. In this tiny quantity of radium bromide,
weighing half a grain, we know with fair certainty there
are fifty million billion (5 x lO-"^®) separate atoms
of radium, assuming that the compound is pure. It
has been proved that, roughly, one two-thousandth of
these disintegrate yearly. There are about 32,000,000
seconds in a year, so that in every second of time
rather less than one thousand million of these radium
atoms disintegrate, giving some small multiple of this
number of a- and /3-particles. So mighty a host pro-
jected outwards in all directions at random, as you may
suppose, fill the surrounding space with their trajec-
tories to all intents and purposes as completely as if
RESOLUTION OF a-RAYS 41
they advanced as one continuous spherical wave-front.
In other words, if only the number of projected particles
is sufficiently great a discrete radiation will be, in many
of its general effects and laws of propagation, not dif-
ferent from a wave-radiation. It is true that such a
radiation will show neither regular reflection, refrac-
tion, nor polarisation in the manner that light does,
and the absence of these phenomena for the a- and /3-rays
is part of the evidence in favour of their discrete nature.
If, however, we continuously reduce the number of
particles ejected, in other words, if we continuously
diminish the quantity of radium employed, there should
come a point when the discrete radiation should no
longer simulate the wave-type. It should, as it were,
break up and show discontinuity, much as some of those
faint continuous light-patches in the heavens, known
as the planetary nebulae, when investigated by more
and more powerful telescopes, begin to break up and
show discontinuity, and finally are resolved into an in-
numerable host of separate twinkling stars. Is it possible
so to resolve a swarm of a-rays ?
The Individual Atom of Matter.
The older physicists who first deduced by accurate
computation the weight and measure of the single indi-
vidual atom and evaluated the number of billions con-
tained in the smallest portion of matter perceptible to
the senses, had they been soberly asked whether it
would be possible ever to observe a single atom of matter,
would have scouted the bare possibility. A single atom
of matter ! A single atom of matter ! I recall this
one exclamation, repeated over and over again with
varying intonation by a distinguished foreign visitor,
whose years had been spent at the microscope on the
borderland between the perceptible and the imper-
ceptible worlds, when the question we are now consider-
ing was under discussion at a British Association
meeting.
42 RAYS OF RADIOACTIVE SUBSTANCES
Let us, however, now make a few calculations to see
whether there is any hope whatever of being able to de-
tect the effect of, say, a single a-particle expelled from
radium, in the same sense as it has been found possible
in astronomy to detect the individual stars which go to
make up a planetary nebula.
In an earlier lecture (p. 17) I alluded to the smallest
quantity of radium that could be detected by the aid of
the gold-leaf electroscope, that is, therefore, by means of
the a-rays emitted. It was one three-thousand-millionth
of a grain. Half a grain, as we have seen, gives out a
few thousand million a-particles every second. So that
the smallest quantity of radium detectable by the
ordinary electroscope must be giving out only a few
individual a-particles per second. From a very early
stage it appeared not inconceivable to Rutherford that
a discontinuity in the emission of a-rays might actually
be detected by using a very minute quantity of radium.
The Spinthariscope.
The problem was actually solved, almost unawares,
by Sir William Crookes, by means of an instrument he
devised and called the Spinthariscope. The instrument
is the only genuine instrument worked by radium that it
is at present possible to buy at the optician's in the ordi-
nary way, and it can be bought — radium and all — for a
few shillings. The reason for this apparent paradox is
to be found in the fact that it is in the essence of the
result to be attained to reduce the amount of radium to
the smallest possible quantity, and this unusual condi-
tion allows of a practically unlimited number of spin-
thariscopes to be made out of an almost invisible
quantity of radium bromide. The amount of radium in
each instrument is absolutely unweighable and invisible.
A needle, A, is made to touch a tiny phial which once
contained radium, and is then mounted (Figs. 10, and 11
facing p. 35) centrally in a little brass tube, the size
of a small reel of cotton, at the bottom of which is a
THE SPINTHARISCOPE
43
phosphorescent screen, B, coated with zinc sulphide.
At the other end of the tube is a lens, C, for magnifjdng
the screen and, by means of a little screw, D, outside,
the needle point may be moved nearer to or away from
the screen. If now in a dark room the screen is ob-
. served through the lens, it will be seen to be luminous,
and this luminosity can be concentrated or spread out
by screwing the needle point nearer to or farther from
the screen. After the eye has become used to the dark-
ness it will be seen that the luminosity is not just a quiet
Fig. 10.
continuous glow. The hght, like that of the planetary
nebulae, has been resolved and shows discontinuity.
It resembles most nearly a shower of shooting stars.
Bright momentary flashes of light or scintillations, too
numerous at any instant to count, are appearing and dis-
appearing in the field of vision. These flashes are caused
by the a-particles of radium. This minute insignifi-
cant trace of radium is positively belching forth a-
particles. It seems incredible that the incessant bom-
bardment of the screen can be caused by such an in-
finitesimal amount of radium. Yet so it is, and in a
month's time, if the instrument is re-examined, it will
44 RAYS OF RADIOACTIVE SUBSTANCES
be found that the scintillations are as numerous and as
brilliant as formerly. After a time, perhaps a year, the
phosphorescent screen itself will be worn out by the
incessant bombardment, will become insensitive and
need renewal. But replace it by a new one and the
radium will be found to be as energetic as ever. The
owner of the instrument will pass away, his heirs and
successors, and even his race will probably have been
forgotten before the radium shows any appreciable sign
of exhaustion.
The actual a-particle itself must, of course, be ex-
tremely small. How else could a mere speck of radium
send out such an incessant and numerous swarm ? As
we have still to prove, the a-particle is an atom of helium,
the second lightest atom of matter known. A grain of
radium bromide expels every second about ten thousand
million a-particles, and if we contemplate this mighty
swarm expelled once every second of time throughout
many centuries we may begin to have some idea of how
many atoms there must be in a single grain of matter,
and how small must be the single atom. The philoso-
phers of only a generation ago would have ridiculed the
hope that we should ever be able to look through a
magnifying-glass to see the effect of a single atom of
matter, yet each of the scintillations of the spinthari-
scope is nothing else.
Decay of c-Radiatign.
The spinthariscope was the original, but to-day it is
only one of many lines of evidence which have estab-
lished the discrete character of the a-radiation and the
nature of the a-particle. We know of many radioactive
substances — polonium is one — emitting a-radiations,
which gradually and completely lose their radio-
activity with the lapse of time. Anticipating, we may
say that the disintegration of polonium proceeds so
rapidly that it is complete in the course of a few years.
Were the process at all similar, for example, to the case
COUNTING a-PARTICLES 45
of a hot body cooling, one would expect a gradual altera-
tion in the character of the radiation with the diminu-
tion of its intensity with lapse of time; whereas the
character of the radiation is exactly the same at the end,
when it has nearly all decayed, as it is at the beginning.
This is explained simply on the view that the number
of a-particles expelled grows less as the activity decays.
The individual a-particles have the same velocity and
other characteristics, whether expelled at the end or at
the beginning of the process. Professor Bragg' s dis-
covery that each a-particle has a definite " range,"
characteristic of it, is quite inexplicable on a wave
theory. The range of the a-particles emitted by
polonium, for example, is thirty-eight millimetres of
air, and though in the course of a few years the a-radia-
tion of polonium decays always completely, the range
of the a-particle expelled at the end is exactly the same
as at the beginning.
Counting the a- Particles.
In this connection, finally, I may mention some really
wonderful work recently done by Professor Rutherford
and his co-worker Dr. Geiger, in which they have actually
succeeded in counting directly the number of a-particles
expelled from a given quantity of radium every second.
As you may know, if two points are connected to an
electrical machine, or other method of generating
an electric force or tension, a spark will pass between
them under suitable circumstances. Now suppose the
distance apart of the two points is just so great
that no spark will pass with the particular electrical
tension applied, and that some radium is then brought
near to the points. Then a spark will pass. The rays
from radium by making the air a conductor of electricity
facilitate the passage of the spark, so that under their
influence the discharge will leap across a greater distance
than it otherwise would. Substitute for the crude
method of detecting the discharge by means of a spark
5
46 RAYS OF RADIOACTIVE SUBSTANCES
a highly refined electrical instrument, known as the
electrometer, in which, as in the galvanometer, a spot
of light is reflected from a mirror attached to a needle,
which can be arranged to move when a discharge passes
across the gap, and you have the essential principle
of Rutherford's arrangement. Such an arrangement
can be made so excessively sensitive that the passage
of a single a-particle from radium through what cor-
responded to the " spark gap " of the first arrange-
ment described, is sufficient to cause the spot of light
from the needle of the electrometer to move with a
sudden jerk. The experiment consists, then, in counting
the number of these sudden jerks of the electrometer
needle in a given time, when a known quantity of radium
is placed at a known distance. The radium has to be
placed many yards away from the apparatus, and the a-
rays are fired along a long exhausted tube with a small
window at the end to admit the passage of a very minute
definite proportion of the total number of a-particles,
which proportion can be calculated. In the actual ex-
periments the distance of the radium and the size of the
window through which the «-particles passed were such
that, roughly, only one out of every 100 million a-
particles expelled found their way into the apparatus.
The total number of a-particles actually expelled per
second by a grain of radium in its normal condition was
found to be about ten thousand million. Per milligram
of radium the exact number per second is 136 million.
These results were also checked by counting the number
of scintillations per second in a special form of spinthari-
scope. There have always been scientific men who have
regarded the atom and the atomic theory with suspicion,
and have never tired of insisting upon its " hypothet-
ical " character. It may therefore be rightly regarded
as one of the greatest triumphs of science that an ob-
server can now actually sit down in front of a vessel and
with the aid of a watch count the number of atoms
entering it every minute from a quantity of radium
outside.
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To face p. 47
CHAPTER IV
RAYS OF RADIOACTIVE SVBST AN CES— Continued.
The y8-RAYS.
In addition to their varying power of penetrating
matter, there is another test which has proved of great
service in analysing the three types of rays from radio-
active bodies and in deterhiining the real nature of each.
The trajectories of some of the rays are powerfully in-
fluenced by a magnet, others are slightly, and others not
at all affected. Thus the ;8-rays of all radioactive sub-
stances if caused to traverse the space between the poles
of a magnet are very strongly deflected, and if the magnet
is a powerful one may be completely coiled up into closed
circles or spirals.
Faraday imagined that between the N-pole and S-
pole of a magnet there existed actual lines of magnetic
force. In the electro-magnet on the table (Fig. 12),
which is formed so that the N- and S -poles are bent
round so as to face one another, the lines of force between
the opposite faces of the two pole-pieces are straight
lines following the shortest distance between them. It is
convenient to imagine with Faraday the actual existence
of such lines of force. An electro-magnet is simply an
arrangement in which a bar of soft iron can be magne-
tised at will by passing an electric current through a coil
of wire wound round it. Soft iron of good quality, unlike
steel, retains no appreciable permanent magnetism. It
is very easily magnetised by an electric current, and
its magnetism continues just so long as the current,
and ceases practically completely when the current is
switched off.
47
48 RAYS OF RADIOACTIVE SUBSTANCES
Deviation of ^S-Rays by a Magnet.
Now suppose a beam of ;S-rays of radium to be fired
through the space between the pole-pieces at right
angles to the lines of magne ic force. The path of the
rays is bent. The rays tend to coil round the magnetic
lines of force in circles. Suppose we look along the lines
of force stretching from the N-pole to the S-pole, that
is to say, suppose the eye to be placed at the centre of the
N-pole and to be looking towards the centre of the S-pole.
Then the yS-rays will be coiled round into circles in a
direction of rotation opposite to that of the hands of a
clock, that is, as we say, counter clock- wise. If we look
from the S-pole to the N-pole the direction of rotation is
clock- wise. Now if the radium is placed behind the poles
of the electro-magnet, and a screen of platinocyanide of
barium is placed in front, and the distance between them
is so adjusted to the strength of the magnet that when the
latter is excited by an electric current the /3-rays from
the radium are coiled up into circles of lesser diameter
than the distance between the radium and the screen,
none of the /3-rays will now reach the screen. This will
be seen from Fig. 13. In this figure the eye is supposed
to be at the centre of the S-pole of the magnet, looking
towards the face of the N-pole. The rays from the
radium passing up between the N-pole and the eye,
in the top diagram, reach the screen. In the lower
diagram the magnet is in action, and the rays are coiled
clock-wise into circles, none reaching the screen.
The radium is contained in its mica- covered capsule
so that only the ^- and 7-rays are dealt with, the a-rays
being suppressed. In the darkness you see the phosphor-
escent screen brilliantly luminous so long as the magnet
is not excited. I switch on the current and the light of
the screen at once goes out almost completely. The
faint luminosity left behind is due to the 7-rays, which
are not deviated at all, so far as we know, even by the
strongest magnetic forces. If I interpose a penny in
MAGNETIC DEVIATION OF yS-RAYS 49
front of the radium so that the 7-rays have now to
traverse it before reaching the screen the faint lumin-
osity is hardly diminished. Now I switch off the excit-
ing current and the magnet almost instantly loses its
magnetism, the /3-rays spring back out of their circular
into straight trajectories, strike the screen and cause it
SCREEN
Magnet off.
SCREEN
Magnet on.
Fig. 13.
again to flash out into brilliance. Now the introduc-
tion of a penny causes the luminosity practically to
disappear, all but for the faint glow due to the 7-rays.
Electric Charge carried by /S-Rays.
To a trained physicist the interest of this behaviour
is due to the fact that it is exactly what would happen
to a current of electricity if it were made to flow between
50 RAYS OF RADIOACTIVE SUBSTANCES
the poles of a strong electro-magnet. If we employed
a piece of ordinary wire to carry the current, the wire
would tend to coil up into a circle exactly like the /S-ray,
and there would be a battle between the natural stiffness
of the wire and the deviating magnetic force, and it
would depend on their relative strengths which prevailed.
With care, however, it is possible to use a fluid wire,
which has no stiffness. If a strong current is passed
through a thin aluminium wire it, of course, gets hot
and finally melts, but retains its original form without
breaking, hanging by virtue of its weight as a beautiful
loop of glowing molten aluminium. Such a loop pro-
vides an extremely sensitive means of investigating the
laws of action of magnets on currents, and you can see
how violently and powerfully it is deviated if it is hung
between the poles of the electro-magnet and the magnet
then excited. The iS-rays, as they traverse their course,
behave exactly like a current of electricity. If they con-
sisted of extremely rapidly moving particles — charged
with electricity — we know that such a stream would
behave to a magnet exactly like a current flowing in a
flexible conductor.
The Nature of Electricity.
Now we do know the direction in which the ^-rays
are moving, namely from the radium, but we do not
know, or at least did not till recently know, the direc-
tion in which the electricity is moving in an electric
current. However, by long usage we speak in a purely
conventional way of the + and - ends of a wire in
which a current is flowing. We do not yet know
for certain whether there are two kinds of electricity,
a positive kind and a negative kind, but the probability
is that there is only one kind, the negative kind, and that
the effects of the opposite kind are due to a relative
electrical scarcity or vacuum. It is much the same with
heat and cold, except that we know the real thing is
heat, and cold is the absence of it. A trained physicist
POSITIVE AND NEGATIVE ELECTRICITY 51
will speak of so much heat, or so little heat, or or one
body having so much less heat than another, but he will
not speak of so much cold, or one body having more
cold than another, although often such a method of
expression would be convenient and would lead to no
error. In this sense we may speak both of positive
and negative electricity without error. A current of
electricity flowing along a wire from the positive to the
negative we may look upon as due to the transport of
positive electricity in the direction from + to -, or as
the transport of negative electricity from - to !- . The
two ideas are equivalent and, in fact, identical for the
present purposes. On the view that there is only one
kind — the negative kind — of electricity, a positively
charged body or atom is merely a body or atom with less
negative electricity than is normally present in an " un-
charged " or electrically neutral body.
In the /S-rays we have a movement of charged par-
ticles/rom the radium, and we have to find out whether
the particles are positively or negatively charged, using
the terms positive and negative in their conventional
electrical significance. If the rays were deviated in the
same sense as a current flowing from + to - in the same
direction as the rays, obviously we should conclude the
/3-rays were + ly charged. As a matter of fact we
find the opposite is the case. When the yS-rays
are deviated clock-wise by the magnet a current of the
kind described would be deviated counter clock-wise.
To simulate the deviation of the /S-rays the electric
current must be a negative current, that is to say, must
be either negative electricity flowing in the direction of
the rays, or positive electricity flowing in the opposite
direction. As there is no reason to doubt that the
y8-rays do come from the radium, the electric charge
they carry must be negative.
Modern views are definite on the point that if there is
only one electricity, that one is the kind which by con-
vention has, unfortunately, been styled negative. The
negative is the real electricity. The positive may, like
S2 RAYS OP RADIOACTIVE SUBSTANCES
cold, be the mere deficit of the real kind, or it may have
a separate existence, the mirror image as it were of the
other kind. I personally have always preferred the
view that negative electricity is " electrical heat " and
positive electricity " electrical cold," but a real answer
to this question would no doubt prove itself to be a very
fundamental step, and would require much further con-
sideration.
The behaviour of the /3-rays in a magnetic field as-
sociates them at once with some previously known
radiations from the electrical discharge tubes exhausted
to an extremely high degree of vacuum which are known
generically as Crookes' tubes, from their first systematic
investigator. Into this field of work I have no intention
of entering in detail, for it is the one aspect of this subject
which has received the most adequate treatment in the
accounts of radioactivity written for the benefit of the
public. A brief resume only must suffice.
Radiant Matter or Cathode-Rays.
The ^-rays are very similar in nature to the " Radiant
Matter " (also called " cathode-rays " or " cathode-
streams ") of Sir William Crookes, obtained when an
electric discharge or current is passed through a vessel
exhausted to a very high degree of vacuum. The requi-
site degree of vacuum can be obtained with a little trouble
by the aid of a mercury pump, based on the same
principle as Torricelli's celebrated experiment with the
barometer. But far quicker and more efficient methods
have lately come into use. One such consists in absorb-
ing the last traces of gas in the pores of the charcoal of
cocoa-nuts cooled to the temperature of liquid air,
according to the discovery of Sir James Dewar.
Another method consists in absorbing the last traces of
gas with the vapour of metallic calcium heated to a very
high temperature in a special vacuum furnace. The
discharge from the cathode, or negative pole, in a high
vacuum then consists of radiant streams of particles
RADIANT MATTER 53
travelling in straight lines and producing vivid green
phosphorescence where they strike the glass walls of the
vessel. Any obstacle placed in their path casts a sharp
shadow, the glass beyond not fluorescing where pro-
tected from the bombardment by the obstacle. These
particles also carry charges of negative electricity, and
have great energy, heating to whiteness a piece of
platinum interposed in their path, and causing the most
intense fluorescence of willemite in the same way as the
radium rays. Like the /5-rays they are deviated by a
magnet, and in the same sense, only very much more
easily. Here is a form (Fig. 14) of Crookes' tube,
designed to show the cathode-rays and their deviation
by the action of a magnet. The electrodes consist of plates
of metal, which are attached to the terminals of an
induction coil, or an electrical machine or other suffi-
ciently powerful source of electric tension. One elec-
trode A is connected to the positive pole, and the other
electrode B to the negative pole of the coil, and we have
to concentrate our attention on the negative electrode,
this being what is also called the " cathode." The tube
has been exhausted by a pump until there is only about
one ten-thousandth of the air left, and was then sealed
up. Under such conditions the glass of the tube shines
with a brilliant fluorescence when a discharge is forced
through it. This fluorescence has been traced to " rays "
inside the vessel, proceeding from the cathode at right
angles to its surface and travelling in straight lines
through the tube. Wherever they strike the glass they
cause it to glow, just as the radium rays do.
In front of the cathode is a piece of mica with a slit cut
in it, which stops all the rays except a narrow pencil
passing through the slit. Along the length of the tube is
fixed a fluorescent screen in the form of a plate painted
with powdered willemite, and as the narrow pencil of
rays impinge on this plate they trace out their path as a
bright line of green fluorescence. Now if one pole of a
magnet is brought behind the tube the rays are bent
sharply to the left or right, depending on whether the
54 RAYS OF RADIOACTIVE SUBSTANCES
a
cn
00
Fig. 14.
THE ELECTRON 55
N- or the S-pole of the magnet is presented to the tube.
The direction of the deviation is the same as with the
/3-rays, and before even the ;g-rays had been discovered
the cathode rays of the Crookes' tube had been definitely
shown to consist of minute particles charged with
negative electricity flying off from the cathode with
immense velocity.
The Electron.
What are these particles ? Crookes thought they
were matter in a new or fourth state. To-day we know
they are " electrons." The electron is a new and some-
what startling conception to minds trained on the older
lines, although traces of it date back from the dis-
coveries of Faraday of' the laws of electrolysis. We
owe largely to the well-known investigations of Sir
Joseph Thomson, and his school at the Cavendish
Laboratory, Cambridge, the recognition of the electron
as an atom of electricity, divorced from matter. The
cathode-rays consist of these separate individual and
isolated electrons, repelled out of the metal of the nega-
tive pole under the action of powerful electric stress,
and, in the absence of gas, gathering terrific speed in
their passage through the exhausted tube. Whatever
the manner in which these electrons are produced,
under whatever circumstances they result, they are
always identical in their main characteristics. Their
charge is always the same, and also their " mass,"
although their velocity may and does vary according
to the conditions within very wide limits. They and
their motion are responsible for the most varied and
apparently unconnected phenomena in Nature, and in
the empire of matter they seem often to occupy a r61e in
comparison with the more massive material a':oms
analogous to the part played by the planets in relation
to the central sun of a solar system. The mass of the
electron is only one two-thousandth part of that of the
hydrogen atom, the smallest particle previously known.
The methods employed depend upon tracing the
56 RAYS OF RADIOACTIVE SUBSTANCES
paths of the cathode-rays when they were subjected
simultaneously to electric and to magnetic fields.
Both fields deflect the cathode-particle but in different
ways, and from the results the charge, mass, and
velocity of the particle were separately found.
Inertia or Mass.
In some ways we know far more about the electron
than about the atom of matter. The electron cannot
move without disturbing the medium which occupies all
space continuously, and which we, not yet knowing too
much about its real nature, call the ether. It is the
motion and change of motion of the electron which give
us light, the X-rays, and the long ether waves used in
wireless telegraphy. It is the reaction of the ether on
the moving electron which gives it its " mass." Now
this " mass " of the electron, applied as the term was to
the atom of pure electricity entirely unassociated with
matter, needed very careful and clear thinking, or it
would appear utterly contradictory to the older concep-
tions of matter. The term mass, used in this sense, has
nothing to do with the effect of gravity or weight, as it
is still absolutely unknown whether electrons obey the
law of gravitation.
In this region of new ideas we are now entering, more
difficulty, perhaps, is to be anticipated in the meaning
attached to the terms employed than in the actual ideas
themselves. Mass is often equivalent to " weight," but
here it is not so. The mass of, meaning the quantity of,
matter, is a fundamental idea, while weight is a derived
idea due to the earth's attraction. A given quantity of
matter throughout the universe has an unchanging mass.
Its weight, of course, depends upon the proximity and
ma ;s of the world attracting it. What then is the measure
of mass as distinct from weight ? Weight is, as a matter
of fact, invariably used on the earth to measure mass
because it is so convenient. Yet if we can imagine our-
selves isolated in space at a great distance from all worlds
INERTIA 57
with a given quantity of matter it is desired to know the
the mass of, we should still have no difficulty in dis-
tinguishing the greater mass, say of a sphere of lead,
from the lesser mass of a similar- sized sphere of wood.
We should know the difference by the difference of
inertia. If we struck each a similar blow the wood
sphere would start to move many times as fast as the
lead sphere. Neither would have appreciable weight
under these circumstances, but their relative inertia
would still be in proportion to their masses. A collision
between two " weightless " railway trains meeting in
mid-space would work just as much havoc to the trains
as it would if it occurred at the same speed upon the
earth. Hence when a physicist speaks of the " mass "
of a/3-ray particle, or of a cathode-ray particle, no con-
siderations of weight are in his mind.
Sir J. J. Thomson, first with these cathode-rays, after-
wards with the /3-rays, showed how it was possible, by
measuring the extent to which they were deviated by
magnetic and by electric forces, to determine the
velocity, the charge, and the mass of the particles which
constitute them.
The application of these methods resulted in the proof
that the charge and the mass of the ;8-particle were
identical with that of the cathode-ray particle of vacuum
tubes, but the velocity of the iS-particle was far higher
than that of the fastest known cathode-ray. Thus the
/3-particle ejected from the radium atom was already
known. It is true it is ejected more violently by radium
than in any previously known case, but in its essential
characteristics, its charge, or the quantity of electricity
it carries, and its mass — it is the same particle as Sir
William Crookes dealt with in his vacuum tubes thirty
years ago. He christened them in a prophetic moment
with the name of " Radiant Matter," and was, like many
another prophet, ridiculed for his pains.
58 RAYS GF RADIOACTIVE SUBSTANCES
Velocity of the ^-Rays.
The cathode- ray particle, and also the y8-ray particle,
were found to carry the same amount of electricity as
the charged hydrogen atom. Hence, whatever else the
/3-particle of radium is, it is certainly an atom of nega-
tive electricity. With regard to the velocity, just as the
mass of these particles is smaller than any known
material particle, their velocity is appropriately almost
inconceivably greater than that of any previously known
material particle. It approaches that of light itself,
which has a velocity of 185,000 miles per second. The
average velocity of the cathode -ray particle of the
vacuum tube is from 5,000 to 10,000 miles per second ;
while that of the fastest of the /3-particles of radium is
so nearly that of light as to be indistinguishable from it.
Most of the /3-rays, however, travel with a velocity from
40 to 80 per cent, of that of light.
This is one of the most general, as it is one of the
most remarkable, features about radium. The effects
produced by its rays, even the rays themselves in some
part, are not entirely new. They can be simulated to
some extent by artificial means. In passing from the
effects produced artificially to those produced by radium
spontaneously, we are aware of great resemblances, and
at the same time of great differences. By the use of
exceedingly powerful electrical appliances, and the ex-
penditure of a considerable amount of energy, we can
simulate to some extent the ;8-rays of radium, but no
instrument maker at the present time can provide you
with the means of impressing upon the artificially
generated cathode -ray electron of the Crookes' tube more
than a small fraction of the velocity with which the
/3-ray electron is being spontaneously expelled from
radium. It is the same in other matters. The utmost
we are able to effect by the most powerful forces at our
disposal falls far short of what is being done spon-
taneously by a mere speck of matter undergoing atomic
disintegration.
A PERPETUAL MOTION MACHINE
59
The Radium Clock.
Before leaving the subject of ^g-rays, I have to show
you an interesting instrument devised by Professor
Strutt/ and popularly called the radium clock
(Fig. 15). It is the nearest approach to perpetual
motion that has yet been devised, and it consists of a
gold-leaf electroscope, worked by the
negative electricity carried away from
the radium by means of the ^-rays.
A few milligrams of a salt of radium
are contained in a thin-walled closed
glass tube, A, through which the
j8-rays can easily penetrate, and this
tube is supported from an insulating
rod of quartz, B, within a highly
exhausted glass vessel. The tube in
turn carries at its lower end two gold
leaves, C, after the manner of an
electroscope. The yS-rays shot out
from the radium carry away negative
electricity, and therefore the radium
itself left behind becomes positively
charged. The gradual accumulation
of this charge causes the gold leaves
attached to the tube to diverge little
by little, until they touch the sides of
the vessel and are discharged, when the
cycle of operations recommences. The
instrument on the table (Fig. 16,
facing p. 47) was constructed many
years ago, and has been functionating about once every
three minutes ever since. There is no reason why it
should not do so for at least a thousand years more,
though at a slowly decreasing rate. Though not a true
perpetual motion machine, it is one so far as only our
lives are concerned.
Fig. 15.
Now Lord Rayleigh.
60 RAYS OF RADIOACTIVE SUBSTANCES
Magnetic Deviation of a-PARTicLES.
The methods we have been considering which led to
the elucidation of the real nature of the iS-rays — the
determination of the nature of the expelled particle, its
mass, charge, and velocity — have been applied success-
fully also to the elucidation of the real nature of the
a-rays, though here the task was very much more
difficult experimentally. Rutherford, to whom we
owe our knowledge of this subject, worked for a long
time before he could detect any influence produced
by the most powerful magnets on the course of the
a-rays, so slight and insignificant it is compared with
the effect on the ^-rays. Finally, he proved that the
a-rays were deflected both by electric and by magnetic
forces, but to an extent of the order of one-thousandth
part of the effect that the /3-rays suffer under similar
circumstances. The deviation of the a-rays, moreover,
is in the opposite direction to that of the ^S-rays. Where
the yS-rays are coiled clock-wise, for example, the
a-rays would tend to turn counter clock-wise. By
these, and numerous other experiments, it has been
shown that the a-rays consist of positively charged
particles. The a-particle is, however, not, like the
yS-particle, only a disembodied electrical charge. It is
a charged material atom. At first it was thought to
be twice as heavy as the hydrogen atom, on the assump-
tion that it was charged with a single " atom " of positive
electricity. Now, however, it has been proved to carry
two charges of positive electricity, and to be an atom
four times as heavy as hydrogen. This is in accord with
the whole of the rest of the evidence of radioactive
changes still to be considered, which points unmistakably,
though indirectly, to the conclusion that the a-particle
is an atom of the element helium. The atomic weight
of helium is four, or, in other words, the helium atom
is four times as massive as the hydrogen atom, which
is always taken as unity. In our most recent view,
THE ENERGY OF THE a-PARTICLE 61
to be later considered, the a-particle is the nucleus
of a helium atom that has lost the two electrons that
accompany it as satellites in the normal " uncharged "
atom.
Velocity of the a-PARTiCLE.
Waiving the case of the /3-rays which, as we have
seen, are electrical rather than material in nature, the
a-rays of the radioactive substances furnish without
doubt one of the most wonderful phenomena at present
known. If radium did nothing else but send out these
a-particles, that alone would of itself constitute a new
epoch in our knowledge of nature. Take their velocity,
for instance, which, though lower on the average than that
of the ^-rays, reaches in spme cases the very handsome
value of over 12,000 miles per second. This is hundreds
of times faster than the next fastest known material
thing moving in earth or air or space. The swiftest
flight known previously is that of some of the shooting
stars, which attain sometimes to a speed of from twenty
to forty miles a second, and from the attack of which we
are largely protected by the fact that their velocity is so
great that they are quickly dissipated into vapour by
the simple resistance of the air. While such a meteor
was traversing the distance to the moon an a-particle
would, given an unimpeded path, reach the sun.
Such a velocity multiplied by itself, or squared, gives
us a measure of the energy possessed by the a-particles.
If their velocity is, say, half a thousand times faster
than any previously known, the kinetic energy they
possess is, mass for mass, a quarter of a million times
greater than any we have ever had to do with before.
In this fact lies the key to many of the surprising revela-
tions of radium. When we speak of being able to detect
the effect of a single a-particle, and therefore of a single
atom of matter, we mean the detection of its energy,
which is a quarter of a million times as great as that of
any other kind of atom known to us. Similarly, when we
speak of being able to detect in a few seconds by radio-
62 RAYS OF RADIOACTIVE SUBSTANCES
active methods the course of a change which would have
to proceed continuously for geological epochs before it
produced an effect detectable by the most sensitive
chemical test, it is because, firstly, we detect the energy
evolved by the change, not the change itself ; and,
secondly, because the energy is at once so relatively
enormous and at the same time so much more easily
detected compared with any other kind of energy out-
burst previously known to us.
Passage of cu-Particles through Matter.
Matter moving with the speed of 10,000 miles a second
is so novel and strange to us at present that it is doubtful
whether our ordinary conceptions afford much guide or
analogy. The muzzle-velocity of a cannon-ball, for
instance, is a small fraction of one mile per second. Now
we have seen that the a-particle of radium is capable of
traversing very thin aluminium leaves and also several
inches of gaseous air. It is extremely interesting to
inquire what happens during the collision of an a-particle
with a molecule of gas or metal. Some at least of
these collisions must be full and direct, not simple
grazing or glancing coincidence ; and it seems at first
sight difficult to believe that an a-particle striking a gas-
molecule full and fair should not be stopped, however
fast it is moving. Nevertheless, it is not so. Upon this
matter the researches of Bragg and his colleagues have
thrown a flood of light. His conclusions are as remark-
able as they are definite. " Each a-particle pursues a
rectilinear course, no matter what it encounters ; it
passes through all the atoms it meets, whether they form
part of a solid or a gas (or, in all probability, of a liquid),
suffering no deflection on account of any encounter until,
at any rate, very near the end of its course. ... A thin
metal plate may be placed in the way of the stream,
and so rob every particle of some of its energy, but not
a single one is brought to rest by collision with the atoms
of the metal, and the number of particles in the stream
INTERPENETRATION OF ATOMS 63
remains unchanged." Surely this vivid picture of the
flight of a swarm of a-particles raises anew the old meta-
physical conundrum of the schoolmen, whether two
portions of matter could occupy the same space at the
same time. For the only possible meaning of Professor
Bragg' s conclusion is that the a-particle must go clean
through the atoms of matter it penetrates as though they
were not there, and therefore at the instant of collision
the two atoms do occupy the same space at the same
time. This power of the interpenetration of masses is one
of the peculiar properties of matter moving at these,
what may be termed ultra-material, velocities. We
know for certain it is not a normal property of matter.
The only apparent consequence of the passage of the
a-particle through the atoms it encounters is that it
ionises them, that is, they become charged, some with
+ and some with - electricity, after the collision. It is
probable that the a-particle possesses its charge when it
is expelled from the atom. But whereas in the case of
the ^-particle the charge of electricity is the particle, in
the case of the a-particle the charge would almost
certainly result as a consequence of the velocity with
which the particle is moving, even if it were uncharged
initially. At least it is certain that no atom moving at
10,000 miles a second would continue uncharged. The
very first collision with an atom of matter would " knock
out an electron or two," that is to say, charge the
moving particle positively.
Scattering of a-PARTiCLES,
Since the above quotation was written by Professor
Bragg it has been proved that some of the a-particles do
suffer a deflection or scattering in their passage through
matter. For the vast majority of the a-particles this
deflection is exceedingly slight, but for a very small
proportion of the whole the deflection may be so great as
practically to turn the a-particle back the way it came.
This is extremely interesting. The a-particles alone
64 RAYS OF RADIOACTIVE SUBSTANCES
have access to the real interior of the atom of matter,
and a close study of this phenomenon has resulted in
information being obtained as to what the atoms of
matter consist of. Hitherto science has been completely
confined to the external characteristics of atoms, but
the a-particles, after their passage through these atoms,
will afford some clue, which will be later considered, as
to the nature of the unknown territory which they have
traversed.
The quotation from Professor Bragg (p. 63) pursued
the question of what happens to the a-particle on collision
only as far as the initial stages. Each atom of matter
penetrated robs the a-particle of some of its energy, and
its velocity is therefore diminished as it pursues its path.
But the more slowly it moves the more energy is with-
drawn from it in passing through any given obstacle.
In addition, the slower it moves the more easily is it
deviated from its course, or scattered. In consequence,
the speed is more and more quickly reduced as the end of
its path is approached, and the a-particle thus passes out
of the range of detection somewhat suddenly.
A Method of rendering the Tracks of
k-Rays Visible.
By an ingenious arrangement, C. T. R. Wilson
has succeeded recently in making the paths of many
of the new radiations in air, or other gas, visible to the
eye, and in actually photographing them. These rays
ionise the gas, and leave in their tracks columns of ions,
which are molecules of the gas carrying an electric
charge, and which, although really moving about like
all gaseous molecules at great speed, are, by comparison
with the much swifter rays producing them, almost at
rest. Now these ions, the negative variety more easily
than the positive ions, afford nuclei for the condensation
of moisture from a supersaturated atmosphere. Dust
plays the same part, but all dust can readily be removed.
When moist air in a closed space is suddenly expanded
Fig. 17.
Fig iS.
Fig. iq.
Cloud-Tracks of k-Rays of Radium.
To face p. 6s
TRACKS OF a-PARTICLES 65
the air is cooled and the moisture condenses as mist or
rain on the dust particles^ and carries them down, so
freeing the air from such impurities. If the pure air
is now suddenly expanded within certain well-defined
limits, in the absence of ions or dust, no condensation
is produced. But if the air is traversed by any of the
new ionising radiations, the tracks of the rays, when the
ionisation chamber is suitably illuminated, appear
momentarily as long spider-threads of mist whenever
the air is suddenly expanded and chilled. If a flash of
light is arranged to take place just after the expansion,
the threads may be photographed. In Fig. 17 is shown
the a-rays proceeding from the needle point of a
spinthariscope (p. 43), and in Fig. 18, in the lower part
of the picture, an enlargement of the end of the track
of a single «-particle.
The tracks left by the a-particles are almost all
perfectly straight, but a very few show abrupt
large deflections, and sometimes actually the direc-
tion of travel is nearly reversed. The yS-rays, on the
other hand, give very zigzag tracks. These rays-
are known to be scattered and turned very readily
by their encounter with the molecules of matter,
and owing to the ionisation they produce being less
intense than in the case of the a-rays, their tracks
are much fainter. In the upper part of Fig. 18 is seen
the end of the track of a ^-particle, just before it stops
and ceases to ionise. At first when the yS-particle is
travelling at high velocity its track, which in air may
be several metres long, is very much straighter, and it
may travel for several centimetres without sensible
deflection. The photograph has caught the end of the
track when its energy is feeblest and its liability to be
deviated greatest. In Fig. 19 are shown the photographs
of two a-ray tracks, one the normal practically straight
path, and the other showing two abrupt changes of
direction in its length. To quote C. T. R. Wilson's own
words: "The a-particle has thousands of encounters
with atoms of the gases of the air in each millimetre of
66 RAYS OF RADIOACTIVE SUBSTANCES
its course by which ionisation is brought about, as we
know from measurements made by the electrical method,
and in accordance with this the cloud particles (which
are simply ions magnified by condensation of water)
are so closely packed that they are not separately
visible in the photograph. It is remarkable that only
two encounters out of the many thousands occurring in
the course of its flight should succeed in deviating the
particle visibly from its course and that in these cases
the deviations should be quite large." We shall have
occasion to refer to this again as these phenomena have
thrown much light on the internal structure of atoms.
The experiments have also thrown light on the nature
of the 7-rays, and have made it appear probable that
these rays do not ionise the gas directly, but first
cause the molecules struck to emit a kind of cathode-
or /3-radiation, and it is these secondary radiations
which produce the ionisation.
The Fate of the a-PAHTiCLE.
Fluorescent, photographical, and electrical actions
all cease simultaneously. It is estimated that at the
moment the «-particle ceases to be detectable it is still
moving with the velocity of several thousand miles a
second. For all that is known the particle may then
suffer a sudden stop, or it may continue its course
without ionising the atoms it encounters.
For us who are concerned, for the most part with the
broad limitations of our past and present knowledge, the
most interesting feature of this phenomenon is that it
indicates quite definitely that an a-particle expelled
with an initial velocity below several thousand miles a
second could not by any of the present known methods
be detected. Any of the apparently stable and non-
radioactive elements might be disintegrating and
expelling a-particles, but if these did not attain this
limiting speed we should have no evidence of the fact.
It is really by a somewhat slender margin of velocity
THE VASTNESS OF THE UNKNOWN 67
that the a-particles have come within our knowledge
at all. The light we have gained has but served to
intensify the darkness by which we are surrounded on
all sides. Processes similar to and but little less energetic
than those which produce radioactivity, may be going
on suspected everywhere around us, without producing
any yet detectable effects. Radioactivity is to be re-
garded rather as a benevolent hint given to us by Nature
into secrets we might never have guessed, rather than as
the necessary and invariable concomitant of the processes
of atomic disintegration.
CHAPTER V
THE RADIUM EMANATION
The Source of Radioactive Energy.
If we are to continue to regard energy in the modern
way as something having a definite existence, we have
to answer the question, " From where does the energy
of radium come ?" That it comes from nowhere, or
that it is being newly created out of nothing by radium,
is a view it is not possible to entertain for a moment with-
out destroying the basis upon which nineteenth-century
physical science has largely been reared. ''How has it
got the energy in it to do it ?" is the first question that
naturally arises in the mind with regard to radium, but
obviously we should first ask, " Has it the energy in it ?"
Two Alternative Theories.
If the doctrine of energy is true, there are fortunately
only two possible alternatives to be considered. Either
the energy must be derived from within the radium,
which we shall call the first, and as we think the true,
alternative, or it must be supplied from outside the
radium, and this we shall call the second alternative.
This simple narrowing down of all the possible issues to
two alternatives may appear to you somewhat trite, but
in reality it carries with it far more than appears on the
surface. In the first place, being an intrinsic property
of the element, radioactivity is therefore a property of
the atom, and if we take the first alternative and say
the energy comes from within, it means from within the
atom, and therefore that there must exist an enormous
6S
THE TWO ALTP:RNATIVES 69
and not previously suspected store of energy in matter,
or at least in radioactive matter, in some way inside its
atoms or smallest integral parts.
On the second alternative, which has often been
advanced, radium acts merely as a transforming
mechanism. There are electrical transformers dotted
all over this city, receiving the economically transmitted
but dangerous high-tension currents from the central
power station and delivering the comparatively safe low-
tension currents to your houses, which are wasteful to
transmit for long distances. Are the atoms of radium
acting as the transformers of a mysterious and hitherto
unknown source of external energy, first receiving it and
then delivering it up again in a form which can be recog-
nised ? It may be said at once that so vague a view,
postulating the existence of illimitable and mysterious
supplies of energy from without, cannot be directly
disproved. At first it seemed to provide a way of
escape from some of the more unpalatable logical con-
sequences of the first alternative and was eagerly
adopted. In reality, instead of a way of escape, it
proves to be a veritable will-o'-the-Mdsp, luring on its
followers beyond the limits of credulity into a quagmire
of unsubstantial hypotheses, so bottomless and unreal
that even the facts of radium are a wholly inadequate
justification, and, even so, incapable of throwing any
light on the facts when these are more nearly examined.
Nevertheless, we must pursue both alternatives im-
partially, if only to leave no doubt that both have only
to be fairly considered for one to be dismissed.
On the second alternative the radium owes its activity
to a supply of energy from outside. One has only to
isolate the transformers which light this city from all
connection with the outside central station to plunge the
city in darkness. But we have seen that to quench
radioactivity or to modify it in any way is one of the
things science cannot do. Experiment has proved that
even in the natural state in the mine, hundreds of feet
deep down in the earth, pitchblende exhibits its normal
70 THE RADIU]M EMANATION
radioactivity. So that if it derives its energy from
"s^-ithoiit. this must be of a kind entirely different from
any at present known, for it must be capable of travers-
ing A^-ithout loss hundreds of feet of solid rock. This is
as far as we need pursue the second alternative for the
moment. Provided we can call into existence a new
kind of radiant energy luilimited in amount, permeating
all space and unimpeded by passage through matter of
any thickness, we may, but only so far as we have yet
gone, seek a bare explanation of the energ%' of radium on
the second alternative. Such a xievr would accord at
first sight with the continuous and permanent acti\-ity of
radium for an indefinite time, and there would be no
reason why radioactivity, however intense and power-
ful, should decay or diminish with the lapse of
time.
But if the first alternative is true, and the energ\'
comes from within, large as the store of enevgy in the
atom must be to explain radioactivity, it cannot be
infinite, and therefore it is to be expected that the
activity will slowly decay with the lapse of time. If
two radioactive bodies, one much more powerfully
radioactive than the other, are compared together, it
is to be expected on this xievr that the acti%ity of the
more powerful body will decay faster than that of the
other. But for both a time will come, as soon as
the internal stores of energA* are exhausted, when the
radioactivity will come to an end.
By far the most important consequence of the first
alternative, however, has still to be considered. Radium,
if we call by that name the substance containing the un-
evolved store of energy, can no longer be radium when
the energy- is lost. Coal is not coal after it is burnt.
When energ\-is obtained from matter the matter changes,
and before it can be regained in its former state the
energy evolved must be put back. In no case is it
possible for matter to part with its store of energy and
remain the same, for otherwise you wiU readily see a
perpetual motion machine would be easy enough to
INTERXAL ATOMIC EXERGY 71
construct. Indeed, most of those attempted involved
this impossible assumption.
But we have seen that if the energy is stored up in
the radium it must be -mthin the atom., and, therefore,
if radium changes, it must be a change of the atom and
of the element itself. This change of an element would
be transmutation, which is a more fundamental and
deep-seated change than chemical change or any known
kind of material change, and until the disco ver\' of
radioactivity such changes certainly had never been
observed. If the energ\* of radium comes from within.
radium must be suffering a spontaneous kind of transmu-
tation into other elements. So that, if we would avoid
the necessity of beheving in the process of transmuta-
tion, not as a vague possibiht}'. for example, in the sun
and stars, under some unattainable transcendental
condition, but as actually going on imperturbably
around us, which the first alternative demands, we must
seek a way of escape on the second alternative which
requires none of these bewUdering heresies, but simply
transfers the mystery from the radium to the great
external unknown, and leaves it there in good company
"\\ith many of a similar kind.
The Ixteexal E>rERGY of Matter.
At tliis stage it is well to ask the question. Is there any-
thing opposed either to reason or to probability in the
view that the energy evolved from radium is actually
derived from an existing previously unsuspected internal
store within the atom, and that in this process the
element suffers a transformation into other elements ?
How is it that such enormous stores of energy* in matter
have remained so long unknown ?
One of the most elusive features of energy is that you
cannot say by mere observ'ation. or by the use of any
instrument, how much or how httle is stored up in any
kind of matter. For example, this flask contains a
large quantity of an oUy yellow liquid. We cannot tell by
72 THE RADIUINI EMANATION
simple inspection the amount of energy stored up in this
fluid. It may be some quiet and harmless oil, which can
be shaken vigorously with impunity, or it may be nitro-
glycerine, one of the most dangerous and powerful ex-
plosives. Something more than observation is necessary
to tell us the amount of energy that may be stored within
this substance, possibly only awaiting a slight shock to
be evolved. The only way to find out is to try to
explode it as thoroughly as we can, and then if it will
not explode we may conclude that, as far as we know, it
has no latent store of energy waiting to be loosed from
prison.
Explosion is merelj'' a very rapid and violent type of
chemical change, and the same general idea holds good
for all the changes it is possible for matter to undergo.
AVe may determine the energy evolved or absorbed in
any change, that is, in the passage from one kind of
matter to another kind. We have no means of telling
the absolute amount of energy in any kind of matter.
But the one thing of which the chemist is positive is that
in all the material changes matter undergoes — radio-
activity being excepted— the elements do not change
into one another, but remain in their various compounds
essentially unaltered. If transmutation were possible,
and one element could be changed into another, it would
be easy to measure the difference in the amount of energy
of the two elements.
As it is, the internal energy of the elements remains
always unaffected by pre\aously known material changes,
and therefore till recently quite unknowable.
Before we can find out how much or how little
energy is internally associated with the atoms we
must be able to study a case of transmutation. The
great stability of all elements under all conditions —
even in the sun the identical elements which we
know here persist, if we can rely on the evidence
of the spectroscope — is well in accord with the view
that all the elements contain a very large store of
internal energy, which is never released in ordinary
INTERNAL ATOMIC ENERGY 73
changes, but which makes them indifferent to changes
in their environment. Thus the internal kinetic energy
of a torpedo containing a revolving gyrostat makes
it successfully resist deflection from its course by the
wind and waves. The internal energy of the solar
system, taken as a whole, is the sole reason why it
continues to exist as a system and does not drift
apart.
So far then from there being anything opposed to
reason or probability in the \'iew that the atom of the
element contains a great and hitherto unlaiown store
of internal energy, we see that if it possessed such a store
we could not know of it until it changed, while the greater
the store the more would it resist change from without,
and therefore the less likely should we be to suspect its
existence. From this point forward we shall find that
the more the apparent objections to the first alternative
of internal energy are faced the less serious they appear,
while with the second alternative of external energy the
contrary is the case.
Radium a Changing Element.
Having with these preliminaries somewhat cleared the
ground, I now msh to attempt to explain a series of
experimental investigations which have thrown a flood
of light upon the nature of radioactivity. Though by a
superficial or merely external observation of radium,
even over the period of a whole lifetime, it would hardly
be possible to detect the least change of any kind in the
matter itself or any exhaustion of its output of energy,
these investigations have proved that radium, and every
element that is radioactive, is actually changing in a very
peculiar and definite way. These new changes in radio-
activity are always excessively minute as regards the
actual quantities of matter undergoing change in any
period of time. Except in very special circumstances
they are quite beyond the range of the most delicate
methods of investigation previously known to the
74 THE RADIUM EMANATION
chemist. The methods employed in their investigation
are in the first place wholly novel, but they are none the
less trustworthy or definite on that account.
Disintegration in Cascade.
They depend on the important fact that when a
radioactive element changes it does not as a rule do so
once only, producing in a single step the final product of
its change. Usually there are several successive changes
following one another, so to speak, in cascade. Just as
a waterfall, instead of taking one plunge into a lake,
may cascade in a series of successive leaps from pool to
pool on the way down, so a radioactive element like
radium passes in its change through a long series of inter-
mediate bodies, each produced from the one preceding
and producing the one following. Whereas, however,
the first change is and must be slow, the subsequent
changes may be, and usually are, relatively far more
rapid. But for the existence of these ephemeral,
rapidly changing, intermediate substances, continually
being produced and as continually changing, it is safe
to say the mystery of radium would to-day be still
unsolved.
Picture to yourselves exactly what this problem in-
volves. Out of a remote, and so far as we know un-
limited, past this world has gradually come into the state
we find it to-day, and what we find is that there is a
process knowli as radioactivity still spontaneously going
on in matter in its natural state as it is dug out of the
earth, which we cannot in any way stop or retard, and
which we recognise as the intrinsic property of certain
chemical elements. We must conclude, until we have
evidence to the contrary, that radioactivity is not a
process which has started recently, or that it is confined
to the particular epoch of the earth's history we are now
living in. So long as the radioactive elements have
existed this process must have been going on, and, if we
are forced to the conclusion that the radioactive elements
DISINTEGRATION IN CASCADE 75
are changing, is it not obvious that the changes must be
excessively slow for any of the radioactive elements to
have survived ? What could the methods of chemistry
avail in such a search ? Delicate as these are to-day,
beyond the hmit of what was even conceivable a hundred
years ago, infinitely finer and more sensitive methods are
required.
The geologists tell us, and we shall find in radioactivity
only confirmation, that the earth has existed in much the
same physical condition as it exists to-day for hundreds
if not thousands of millions of years. A chemist could
probably in many cases detect the change of one
thousandth part of one element into another, whereas we
shall come to see that for even such a small fraction of a
primary radioactive element to change a period of the
order of a million years would almost certainly be
necessary.
You all know the stride that chemistry took forwards
when it impressed into its service the spectroscope, and
was able to detect with certainty quantities of new
elements absolutely imperceptible in any other way. For
example, Bunsen and Kirchoff detected by the spectro-
scope the unknown element ccesium in the natural waters
of the Durkheim spring in the Palatinate, but to obtain
enough caesium for their chemical investigations they had
to boil down forty tons of this water. Coming nearer the
present day, Mme. Curie made an equal or even greater
step forward when she impressed into the service of
chemistry the property of radioactivity and discovered
the new element radium in pitchblende, though a ton of
pitchblende contains only two grains of radium. But
we must improve even on this. We have to detect
the change in a minute amount of radium which is
changing so slowly that it appears not to be changing at
all. The actual amount of new matter which this half-
grain of radium bromide would produce by its change in,
say, a month or a year, is a quantity so small that
one has only to attempt to conceive it to be ready to
give up the search in despair. Yet in a moment I hope
76 THE RADIUM EMANATION
to show it to everyone in this large room, and to demon-
strate to you a few of its most striking properties in the
clearest way.
Were radium to change in one single step into, say,
lead, which we believe to be the ultimate product in the
main line of descent, this would be impossible. Those
of you in the back could hardly see a quantity of lead
equal in quantity to the whole of this radium. How
much less then could you hope to be shown the infini-
tesimal fraction of this small quantity which is pro-
duced in a month or a year ? No chemist has yet
detected lead as the final product of radium, and our
evidence on this point is at present only indirect, but it
is now quite conclusive. But radium does not change
all at once in one step. At least eight intermediate
bodies intervene, each one of which is formed from the
one preceding it with an outburst of energy, and changes
into the next with another outburst of energy.
The Successive Outbursts of Energy.
A soldier on a battlefield knows without any doubt
when he is being fired at, but it would take him a long
and patient examination to find out, and it would be a
matter of only secondary interest, whether the bullets
are made, say, of lead or of nickel. The energy pos-
sessed by the flying bullets are their, to him, practically
important feature. After the energy is all spent the
bullet ceases to make its presence felt. So it is with
radium. The energy possessed by the changing inter-
mediate substances and evolved from them is the sole
but sufficient evidence of their existence. After the
energy is all spent and the change is complete, only a
most minute and patient examination, which has still
to be made complete, will reveal the chemical nature
of the minute amount of dead products formed. But
before this stage is reached, in the long succession of
energy outbursts which accompany the change of one
intermediate form into the next, we have a succession of
THE RADIUM EMEANATION 77
most remarkable and obvious phenomena which enable
us to detect the separate changes and to discover the
whole nature and the periods of average life of all the
intermediate bodies, although these all exist only in
absolutely infinitesimal quantity, and not one of them
is known, or probably ever can become known, to the
chemist in the ordinary way. It is one of the most
wonderful triumphs in the whole history of physical
science that such changes should have ever been detected.
Let us turn to the main evidence on which the view that
radium is changing was first based.
The Radium Emanation.
If this specimen of radium bromide was dissolved in
water and the liquid evaporated down to dryness in
order to get back the solid compound, it would be found
that as the result of this very simple operation the
radium had lost the greater part of its radioactivity in
the process. The penetrating 13- and 7-rays would have
completely disappeared, and the non-penetrating a-rays
would only be one quarter as powerful as initially.
Then a strange thing would happen. Left to itself the
radium would spontaneously recover its lost activity,
little by little from day to day, and at the end of a month
it would be not appreciably less active than it at first
was, or as it now is.
This appears to be in direct conflict with the state-
ment previously made that the radioactivity of radium
cannot be affected by any known process, but it is only
apparently so. If we study the process carefully we
shall find that when the radium is dissolved in water
" something " escapes into the air, and this " some-
thing " is intensely radioactive. It diffuses about in the
air, but remains contained within a closed vessel, if it is
gas-tight. In short, this " something " is a new gas
possessing the property of radioactivity to a very intense
degree.
We owe the greater part of our knowledge of this new
78 THE RADIUM EMANATION
radioactive gas to Sir Ernest Rutherford, who has given to
it a special name. He called it the emanation of radium,
or, for short, simply the emanation. The vague term
" emanation " is, with our present exact knowledge of
its real nature, apt to mislead. Some, unfortunately,
have used the term " emanation " or " emanations "
in speaking of the various radiations which radium emits,
and which we have already considered in some detail.
Sir William Ramsay has proposed the name " Niton "
for this new gas, in order to emphasise its relationship
to the other argon gases. However, as similar new
gases or emanations are given by two other of the
radioactive elements, thorium and actinium, the
original term has been generally retained. The term
" emanation," qualified when necessary by the name of
the radioactive element producing it, denotes one of
these new gaseous bodies, and it is necessary not to
confuse this particular use with its older and more
general uses.
Experiments with the Emanation.
In the laboratory, half a mile from this lecture room,
I have a further quantity of about half a grain of pure
radium bromide which has been dissolved in water.
The solution is kept in a closed vessel. This morning
I extracted the emanation from the vessel, and I have
brought it here to show you. The radium from which
it was derived is not in the room, it is still in the labora-
tory half a mile away. The emanation is contained,
mixed with air, in a little glass tube (Fig. 20) provided
with taps for its admission and extraction, and inside
this tube are some fragments of the mineral willemite,
a sihcate of zinc. This mineral has the appearance of
an ordinary cold greenish-grey stone, quite undis-
tinguished and not very different from many of the
common pebbles of the road or seashore. It, however,
possesses the power of fluorescing, under the action of
X-rays and the rays from radium, with a brilliant
Fig. 20. — Tube containing Willemite used to exhibit the Radium Emanation.
Fig. 21. — The Same Tube photographed in the Dark by its Own
Phosphorescent Light.
To face page 78
EXPERIMENTS WITH THE EMANATION 79
greenish light, as you may see when I bring my capsule
containing half a grain of solid radium bromide near to
a block of the mineral in the dark. Let us now in the
dark examine the tube containing the emanation and
willemite together. We find the willemite glowing
with a most remarkable light. Even in ordinary lamp-
light or weak daylight the glow of the willemite is clearly
visible. Fig. 21 shows the tube (Fig. 20), which has
been placed in front of the camera in the dark room, and,
as you can see, the pieces of glowing willemite have
photographed themselves by their own light. In the
negative the walls of the glass tube, which also are
rendered feebly fluorescent by the emanation, are
faintly visible. The photograph proved somewhat
difficult to obtain, as the light, consisting almost wholly
of green and yellow, is almost non-actinic to the photo-
graphic plate. An isochromatic plate must be employed
and a long exposure given. Under these circumstances
the /^- and 7-rays from the tube, as they are not refracted
by the lens, themselves fog the plate uniformly to a con-
siderable extent. The photograph gives no idea of the
beauty of the original tube. Willemite glowing in the
emanation of radium is one of the most beautiful sights
I know, and considered with reference to the origin of
its light and all that the phenomenon foreshadows for
humanity, it raises feelings which only a poet adequately
could express.
What is the emanation of radium ? I shall treat this
question to-night solely as though the emanation was
a body with no connection whatever with radium,
because a knowledge of its own nature is necessary
before its real relation to radium can be appreciated.
In the first place, it is intensely radioactive on its own
account — that is to say, it gives out the new kinds of
rays very similar in character to those given by other
radioactive bodies and capable of producing the same
effects. What I am about to say refers only to a tube
in which the radium emanation has been confined for
some hours. At first the emanation gives only a- but
80 THE RADIUM EMANATION
no ^- or 7-rays, as we shall consider more nearly later
(Chapter IX.).
This tube, in which the emanation is confined, glows
in the dark because the phosphorescent willemite it
contains is being bombarded by the rays from the
emanation. Some of these rays penetrate the glass
walls of the tube, as you may see if I bring the X-ray
screen between your eyes and the tube. Moreover, if
a very thin plate of metal is interposed at the back of the
screen it does not perceptibly diminish the effect, for
the rays from a tube containing the emanation, like the
radium-rays themselves, are capable of penetrating a
considerable thickness of metal. They consist, in fact,
of a-, jS- and 7-rays together. Any of the other phos-
phorescent bodies — for example, zinc sulphide — would,
if placed inside this vessel with the emanation, glow in
its characteristic way just as if exposed to radium itself.
Similarly, a photographic plate would be fogged almost
instantly, and an electrified silk tassel would be dis-
charged at once by the rays proceeding from the emana-
tion confined in this tube. The similarity between the
a-rays from the emanation and those from radium itself
have been proved by exact physical experiments.
The Condensation of the Emanation by Cold.
The next point is that the emanation is not a solid form
of matter dispersed like fine particles of smoke in the air
which carries it. It is a true gas. This has been proved
by innumerable experiments ; but I wish to show you
one which is particularly beautiful, and which has, I
think, convinced everyone who has ever seen it per-
formed that the emanation of radium is a true gas with
the property of radioactivity. It was first performed
by Professor Rutherford and myself in Montreal in
November, 1902. If the emanation is a gas there ought
to be some temperature, though, perhaps a very low
one, at which it loses its gaseous form and is condensed
or frozen. All our attempts to effect such a condensa-
CONDENSATION OF THE EMANATION 81
tion at temperatures down to —100° Centigrade had
proved futile, and we had no means of obtaining the
very low temperatures now daily employed in a modern
laboratory But a liquid air machine was given to the
laboratory by its generous founder, and on its first run
the emanation of radium was successfully condensed.
Exact experiments showed that the emanation is
condensed quite sharply when the temperature falls
below -150° Centigrade (or -238° Fahrenheit), and it
volatilises and again resumes its gaseous state quite
^j^'*^^
Fig. 22.
sharply when the temperature rises above this. We
shall perform the experiment in the following manner
(Fig. 22). To one of the tubes of the vessel containing
the emanation is attached a rubber blowing-ball, for
blowing out the emanation. The other tube is connected
with a U-tube of glass containing some fragments of
willemite, immersed in a vessel of liquid air and so kept
at the very low temperature of about - 183° Centigrade
or - 300° Fahrenheit, into which the emanation is
blown. Exposed to this extreme cold the emanation
82 THE RADIUM EMANATION
instantly loses its gaseous state and condenses in the
tube. To make the experiment more striking, between
the tube containing the emanation and the cooled U-
tube I have interposed several yards of narrow tubing
which the emanation has to traverse before reaching
the tube in which it condenses. As you see, when I
open the taps and gently blow a blast of air to sweep
out the emanation into the cold U-tube, the willemite
in the cold tube suddenly shines out brilliantly, at the
point where the emanation condenses.
So long as the U-tube is kept in the liquid air the
emanation will remain there, though I continue to send
a gentle blast of air from the bellows. But a few
moments after taking the tube out of the liquid air, it
warms up to the point ( - 150° Centigrade) at which
the emanation again resumes its gaseous form, and now
we can blow it out with a single puff of air. See ! I
blow it out through the narrow tubing, which I have
connected to the (J -tube, into a large flask dusted over
its inside surface with the phosphorescent sulphide of
zinc. In the dark the globe shines out with a soft white
light like some fairy lantern, and I can see to read my
watch by its light. The physiological effects of the
radium emanation are imperfectly investigated and may
be potent. This is a field of investigation I personally
have no desire to explore, so that we must not forget to
cork the globe and so prevent the emanation from dif-
fusing out into the air of the room.
The Infinitesimal Quantity of the Emanation.
After this demonstration you may have some difficulty
in really believing that the actual amount of gaseous
emanation which has produced these beautiful effects is
almost infinitesimal. By making use of the same pro-
perty— its condensation by liquid air — the actual volume
occupied by the radium emanation freed by freezing from
all other gases was measured by Sir William Ramsay
and myself. Imagine a bubble of air the volume of a
THE QUANTITY OF THE EMANATION 83
good-sized pin's head, say, one cubic millimetre, or one
fifteen-thousandth part of a cubic inch. It would re-
quire thirty times more emanation than was actually
employed in the last experiment to fill a bubble of this
size. Of course, in the experiments this small quantity
of emanation was mixed with a considerable volume
of air for convenience in manipulation. The actual
quantity of emanation accumulating in a radium pre-
paration is known with accuracy to be 0-6 cubic milli-
metre per gram of radium (element).
It requires a distinct step for the mind to assimilate
the important fact that the property of radioactivity,
which so far has been studied only in solid substances
and minerals, could be shown equally by a gas, and this
fact accounted for the ,true nature of the emanation
remaining largely unrecognised even after the con-
clusive experiment I have shown you. There is, of
course, nothing contrary to the nature of radioactivity
in the fact that it is shown by a gas. When we apply
Mme. Curie's theory that radioactivity is an intrinsic
property of the atom, and of the element in question,
the difficulty is not that the emanation is a gas, for many
elements are gases, but how it is that a new radioactive
element, such as the emanation undoubtedly is, should
result when radium compounds are dissolved in water,
and this question we have purposely deferred.
The Radioactivity of the Emanation.
The emanation, as we have employed it in our
experiments, is mixed with ordinary air, and in this
way it can be dealt with and treated like any other gas.
We have blown it through tubes from one end of the
lecture table to the other. If it had been an ordinary
gas, like air, no one could have seen it, or known what
became of it. But being intensely radioactive, although
its actual quantity is almost inconceivably small, the
radioactivity serves as a sufficient evidence of its
presence or absence, making it, as a matter of fact, far
84 THE RADIUM EMANATION
easier to work with and to investigate than an ordinary
gas in ordinary quantity. If a mining engineer wished
to know how the air he pumped into his mine got dis-
tributed among the various shafts and pits, he could
not do better than to put a little radium emanation into
the entering air, and then subsequently to take samples
at various parts of the mine, and have them tested for
content of radium emanation by a gold-leaf electroscope.
Many other practical problems in the flow of gases,
which are difficult to solve by ordinary methods, might
be readily solved by the help of this new gas.
The Chemical Character of the Emanation.
It has even been found possible to settle the chemical
nature of this new gas, and to place it in its proper
family of elements in the periodic table. Almost all
gases, according to their various natures, are absorbed
when subjected to the action of various chemical re-
agents. Thus oxygen is absorbed by phosphorus,
hydrogen by heated copper oxide, nitrogen by heated
magnesium, and so on. The exceptions, namely, gases
which are not absorbed by any reagents and which will
not combine with anything, are the newly discovered
gases of Lord Rayleigh and Sir William Ramsay — argon,
helium, neon, etc. — which exist in atmospheric air.
The quantity in the air of these gases is extremely
minute except in the single case of argon, which is
present to the extent of one per cent. The radium
emanation, like argon, is not absorbed by any kno\vn
reagent, and does not appear to possess any power of
chemical combination. It may be passed unchanged
through absorbents, or subjected to drastic chemical
treatment which would suffice to absorb every known
gas except those of the argon type, and the conclusion
has been arrived at that the emanation is an element
of the same family nature as the argon gases. Like
them, it exists in the form of single atoms — ^that is,
its molecule is monatomic. Radium, on the other hand,
AN ARGON TYPE OF GAS 85
in its chemical nature is extremely similar to barium,
strontium, and calcium, a family known as the alkaline-
earth elements. None other of the argon elements
or the alkaline- earth elements are radioactive, and yet
the radioactive elements are quite normal in their
chemical properties, closely resembling ordinary ele-
ments, and being associated in the clearest and closest
way with one or other of the old well-known types or
families. More recently, by using quantities of radium
about fifteen times as great as those used to-night in
our experiments, it has been possible to obtain enough
of the emanation for it to be possible to photograph its
spectrum. This proves to be a new and characteristic
bright-line spectrum, resembling in general character the
spectra of the other argon, gases, but absolutely distinct.
It has been found possible to obtain some idea of
the density of the emanation of radium, and therefore
of the weight of its atom, from experiments on the rate
of its diffusion from one place to another. These
indicate that the gas is extremely dense — denser pro-
bably than mercury vapour — and therefore that it
has a very heavy atom. Finally, by means of a new
special micro-balance thousands of times more sensitive
than the most delicately constructed chemist's balance,
the emanation has actually been weighed by Sir William
Ramsay and Mr. Whytlaw-Gray. These experiments
and the whole of the available evidence agree in indicat-
ing that the atomic weight of the emanation is 222,
which is four units below that of radium, and there-
fore is the fourth heaviest known.
The Heat evolved by the Emanation.
The heat given out by a gram of radium, as we
have seen, is 133 calories per hour, but it must be
understood that this refers to radium in its normal
condition containing its full quota of emanation. After
solution in water, that is, after the emanation is ex-
tracted, the radium gives out heat to the extent of only
86 THE RADIUM EMANATION
thirty-three calories per hour, while the emanation
produces one hundred calories per hour. That is to
say, the emanation of radium gives three times as much
energy as the radium from which it is derived, although
the actual amount of matter in the emanation is itself
practically imperceptible.
Now, perhaps it is easy to understand how it is that
the minuteness of the quantities of material offers no
barrier in the investigation of radioactivity. Mass
is not the only consideration. A very small bullet
suffices to work terrible havoc, in spite of its smallness,
by means of the kinetic energy with which it is impelled.
A little torpedo, stuffed full of imprisoned energy in
the form of explosives, suffices to sink an enormous
battleship. A quantity of emanation, which certainly
does not weigh a hundred-thousandth part of a grain,
gives out enough energy to produce effects plainly visible
to you all at the very back of the room.
If, instead of the thirtieth part of a pin's head full,
we could obtain a pint of this gas — and to obtain such a
quantity half a ton of pure radium would be required
— it would radiate the energy of a hundred powerful
arc-lamps. Indeed, as Rutherford has said, no vessel
would hold it. Such a quantity would instantly melt
and dispel in vapour any material known.
The Decay of the Emanation.
These new facts, which transpire the moment we
begin to make a systematic investigation of the radio-
activity of radium, make the second alternative, that
the energy of radium is derived from outside, well-nigh
incredible. For to account for the energy evolved from
the emanation we must suppose all space to be every-
where traversed by new and mysterious forms of radiant
energy of such tremendous and incredible power that
the explanation is harder to believe than the fact it
is supposed to explain. To avoid the necessity of sup-
posing that the energy resides within the comparatively
THE DECAY OF THE EMANATION 87
small amounts of radioactive matter in existence, we
must fill the whole of external space with radiant
energy of a similar order of magnitude. This is strain-
ing at a gnat and swallowing a camel.
Fortunately there is a crucial test by which we are
now in a position to decide between the two alternative
views. Let us apply the theorem we have already
deduced (p. 70) from general principles. If the energy
comes from within the radioactive matter, its radio-
activity must in course of time diminish and decay —
the more rapidly the more powerfully radioactive it is.
Whereas, if the energy comes from the outside, however
powerful the radioactivity may be, there is no reason
why it should not continue indefinitely with undiminished
power.
We have seen that the emanation is, mass for mass,
far more intensely radioactive even than radium, and,
if the energy comes from within, it is to be expected that
the activity of the emanation will be short-lived in
comparison with that of radium, whereas, if the energy
is derived from outside, no such decay is to be antici-
pated. Does the radioactivity of the radium emanation
diminish or decay, or does it continue permanently ?
The answer to this question is that the radioactivity
of the emanation rapidly decays away from day to day.
Four days hence the activity will be but one-half of
what it now is. In eight days the activity will be re-
duced to one-fourth, in twelve days to one-eighth, in
sixteen days to one-sixteenth, and so on, diminishing
practically to zero at the end of a month in a descending
geometrical progression with the lapse of time.
The light from the glowing willemite in this tube,
when it is left entirely to itself, will gradually fade, and
at the end of a month will have died almost completely.
Vast as is the store of energy in matter which is released
in the radioactive process, it is not infinite, and in the
radium emanation we have an example of a change
proceeding so rapidly that only a few weeks are necessary
for its completion.
88 THE RADIUM EMANATION
The Reproduction of the Emanation by
Radium.
Half a mystery is usually greater than the whole, and
in science when mysteries begin to appear on all sides,
the explanation is often near at hand. We dissolved
a compound of radium in water, and the greater part of
its activity disappeared in the process. Then little
by little the lost activity was spontaneously recovered,
and at the end of a month the radium was not appre-
ciably less active than at first. The disappearance of
the greater part of the activity after solution was ex-
plained by the fact that an extremely radioactive gas
— the emanation — was liberated during the act of
solution, and this carried away with it the whole of the
radioactivity which the radium had lost. But, lo !
while the radium slowly recovered its original radio-
activity, the emanation lost what it had at first pos-
sessed. A quantitative examination of these two pro-
cesses of decay and recovery at once showed that the
total radioactivity had not been affected, but had
remained constant in spite of the treatment to which
the radium had been subjected. This is a fundamental
law of universal application to all radioactive bodies,
and it has been called the Law of the Conservation of
Radioactivity. Whatever you do to any radioactive
substance you cannot artificially alter the total radio-
activity, though you may frequently, as in this example,
divide it into several parts, for reasons that will soon
be clear.
It is easy enough on the first alternative to account
for the comparatively rapid decay of the activity of
the emanation of radium. It is dissipating its internal
store of energy so rapidly that it is soon exhausted.
It is a clear case of a short life and a merry one. But
how is the gradual recovery of the radioactivity of the
radium in the course of time to be explaii^ed ? This
is the key to the whole problem, and on the second
REPRODUCTION OF THE EMANATION 89
alternative no answer whatever can be given. The
explanation that the energy of radioactive substances
is derived from outside is not merely incredible. It is
altogether insufficient.
Imagine that a month has elapsed, and that the
radium, which has now recovered completely its lost
activity, is again dissolved in water and evaporated
down to dryness exactly as before. Again you would
find that in the process the radium had lost the same
large proportion of its radioactivity, and again you would
obtain from it a new amount of emanation no less than
that which is on the table to-night. Repeat the experi-
ment as often as you please and you will find the result
always the same. While the emanation you separate
from the radium is decaying away from day to day,
a fresh crop is being spontaneously manufactured by the
radium. The change of the radium into the emanation
is, as a matter of fact, only the first of a long series of
successive changes of a similar character. The gaseous
emanation in turn rapidly changes into a third body,
not a gas, called radium A; this into a fourth, called
radium B; and so on. Nine successive changes are at
present known, which we shall have to give some account
of later.
Atomic Disintegration.
This explanation of radioactivity, which has come
to be known as the theory of atomic disintegration,
was put forward by Professor Rutherford and myself
as the result of a long series of experimental investi-
gations carried out in the Macdonald Physical and
Chemical Laboratories at McGill University, Montreal.
It has, since, not only shown itself capable of interpreting
all the very complicated known facts of radioactivity,
but also of predicting and accounting for many new ones.
Although on the surface a revolutionary addition to the
theories of physical science, it must be remembered that
it is the facts of radioactivity which are really revolu-
tionary. While accommodating these strange new facts
90 THE RADIUM EMANATION
the disintegration theory conserves in a truly remark-
able way the older established principles of physical
science. Without such a guiding hypothesis, recon-
ciling the old and the new, it is safe to say that the facts
of radioactivity would ultimately have wrought a far
greater change in scientific theory than has actually
taken place. Although the emanation of radium is
not and, as we shall come to see, never can be obtained
in palpable quantities — it is changing too rapidly for
that — we know almost as much about its nature and
properties as we do about any of the older gases.
Radioactive Equilibrium.
A very important point is that just as we cannot
really alter the radioactivity of a body artificially in
any way, we cannot and do not in any process influence
the rate at which the emanation is being formed from
radium or the rate at which it in turn spontaneously
changes. The same amount is always in existence
whether you separate it or not. The apparent constancy
of the radioactivity of radium is not the real constancy
to be expected of a transforming mechanism. It is
the apparent constancy produced by the equilibrium
between continuous and opposing changes, on the one
hand the rapid decay of the part of the radioactivity
due to the emanation, and on the other the regeneration
of fresh emanation as fast as the old disappears. This
process of regeneration is always going on at a perfectly
definite and unalterable rate, and the property of pro-
ducing a certain definite amount of emanation in a given
time is as mu( h a part and parcel of the very nature of
radium — and indeed the best and most easily applied
qualitative and quantitative test for the presence of
radium in the minutest quantity that we possess — as
is its power of giving the rays which lit up the X-ray
screen and discharged the silk tassel, or as its power of
generating heat.
ENERGY OF RADIOACTIVE CHANGE 91
Energy of Radioactive Change.
All of these properties are but the various aspects
of a single primary cause. The element radium is
changing, so slowly it is true, that at first sight it appears
not to be changing at all, and yet with so tremendous
and unparalleled an evolution of energy that the trans-
formation of an otherwise imperceptible part of its
mass is accompanied by an amount of energy so great
that the change could not by any possibility have
remained unknown. The emanation is the first main
product of the change of radium. If the emanation
were like lead or any ordinary element it would take
years of accumulation and the most minute and patient
investigation to detect it's production. But it is not.
The emanation changes again into a third type of matter
we have not yet considered (the nature of which does
not yet concern us), but whereas it would take hundreds
of years for any appreciable fraction of the radium itself
to change, the change of the emanation is rapid and
goes to practical completion within a single month.
It is precisely on this account that we can work with
and detect such almost infinitesimal quantities. What
may be termed the material evidence of radioactive
change, the detection, by purely chemical or spectro-
scopic methods, of the materials formed in the changes,
is still scanty, although not altogether lacking. But
the radioactive evidence, which depends not on the
material produced, but upon the energy evolved, and
on the way in which the energy is manifested, is abundant
and sufficient. So long as the energy evolved is suffi-
cient in quantity, and of a kind suitable for detection
in any of the various ways I have illustrated, the actual
quantity of matter producing the energy is of no
consequence.
92 THE RADIUM EMANATION
All Radioactive Changes equally Detectable.
But the amount of energy produced by any change
depends not only on the quantity of matter changing,
but also on the time the change lasts, that is, on the
period of life of the changing matter. Chemical and
spectroscopic methods of detecting matter depend on
quantity, whereas radioactive methods depend on
quantity divided by life. The shorter the life of the
changing substance the less of it is necessary for its
detection by means of radioactivity. This is a merely
preliminary and tentative indication of the operation
of an exactly compensating principle of great importance,
which later it will be possible to formulate as a general
law. Its result in the long run is this. Each of the
ephemeral intermediate substances in the cascade of
changes comes equally within our powers of investigation,
whether it changes slowly or rapidly, whether it lasts
long enough to accumulate in ponderable quantity, or
whether it is changing so rapidly that it
anon,
Like snow upon the desert's dusty face.
Lighting a little hour or two, is gone.
CHAPTER VI
HELIUM AND RADIUM
The Connection of the a-P article with Radio-
active Changes.
Last week we studied the first step in the evidence
that radium is changing, and considered in some detail
the chief practical reason why such changes have proved
within our powers of discovery, namely, that the change
is not single but proceeds in cascade from stage to stage,
producing ephemeral intermediate transition-forms, of
which the radium emanation is one, almost inconceiv-
ably minute in their actual quantity but evolving in
their next change very large amounts of energy, by
means of which it is possible to trace them and study
their nature with ease. We considered the first product
of the change of radium, namely, the emanation of
radium, its nature and properties, and its continual
production from radium. We reserved purposely the
examination of the connection between radium and the
emanation it produces. Now I wish to combine with
the knowledge we have gained of the nature of the radium
emanation that already considered (Chapters III. and
XV.) with reference to the nature of the a-particle.
A radium salt is dissolved in water, and the im-
prisoned emanation, which was formed but stored
during the previous month throughout the whole mass
of the substance, is thereby liberated and escapes.
The radium left to itself continues to produce fresh
emanation at a steady rate. The released stores of
emanation begin to lose their radioactivity. We shall
93 8
94 HELIUM AND RADIUM
confine our attention at first solely to the case of the
radium.
When radium in this way is freed from all previously
formed emanation it still gives out a-particles, although
only now one-fourth as many as it gives out when it
contains its full quota of emanation and other products.
^ These a-particles we regard as pro-
O^^ >«-v duced from the radium atom in the
} "~^ ( ) same change as that in which the
emanation is produced. The emana-
Radium. Emanation. ^. -^ regarded, in fact, as radium
Fig. 23, , , f .-1
that has lost one a-particle.
This, which is a perfectly general point of view, was
proved from the first by the consideration of a mass of
evidence accumulated with reference to the similar
changes going on in the element thorium, but much of
this may be left for later treatment. The evidence
that has since been accumulated enables the same deduc-
tion to be more simply made, and this alone need be
considered. Henceforth the original reasoning as to
the nature of atomic disintegration, although it was,
when first put forward, very complete and convincing
to those acquainted with the whole of the experimental
facts, will be largely replaced by the more direct evidence
since obtained.
Helium and the a-PARTicLE.
We have seen in considering the nature of the a-rays
that they are now regarded as due to the flight of swarms
of helium atoms expelled from the radioactive substance
with an almost inconceivable speed of from 8,000 to
12,000 miles per second. Long before the real nature
of the a-particle was known, helium had been first pre-
dicted to be and then proved experimentally to be a
product of the radioactive changes of radium, and this
chapter in the development of the subject has something
more than an historical interest.
Before proceeding, one underlying consideration
RADIOACTIVE EQUILIBRIUM 95
governing the view that an atom of helium and an atom
of emanation are simultaneously formed when an atom
of radium disintegrates, must be made clear. It refers
to the relative quantities of each product, helium and
emanation, which it may be expected will be formed
by the continuous operation of the process. Helium
we know is not radioactive, and therefore there is no
evidence that helium is changing in any way, and we
may in this sense refer to it as one of the ultimate pro-
ducts of the change. The emanation, on the other hand,
is changing so rapidly that the change may be regarded
as complete in the course of a single month. The bodies
it is changing into we have not yet dealt with, and they
do not immediately concern us.
Now a changing substance, like the emanation,
cannot possibly accumulate in quantity with lapse of
time beyond a certain very small extent. It is true
it is constantly being formed from radium in the same
way as helium, but whereas the helium, being a stable
substance, may be expected to accumulate in a quantity
that is proportional to the time that elapses, the quantity
of emanation will not increase beyond a certain point.
For in a very short time after the process of accumulation
of emanation from the radium begins, as much emana-
tion will itself change as is formed, and the quantity
from that time on will remain constant. This condition
is known generally as " radioactive equilibrium," and
when we speak of the emanation being in equilibrium
with the radium we mean that the quantity of emana-
tion has reached a maximum and does not further
appreciably increase with lapse of time. In the case of
the emanation practical equilibrium results in the com-
paratively short time of a few weeks. That is to say,
however long radium is left undisturbed to accumulate
its emanation, the quantity of the latter never exceeds
a practically almost infinitesimal one, for it is a quantity
which is produced from the change of the radium in
quite a short period of time. Its quantity is therefore
excessively minute. It is so very minute that were
96 HELIUM AND RADIUM
it not changing and evolving energy it would not be
detectable by any ordinary method.
You will see that it follows at once from this point of
view that if any element were produced in the dis-
integration of radium, which itself did not change but
was permanent, then on the one hand, owing to the
extreme smallness of the amount formed, it would not
be easy in a short period to obtain evidence of its pro-
duction, by means of ordinary chemical tests, but, on
the other hand, the quantity would go on accumulating
indefinitely with lapse of time.
The Ultimate Products.
As we saw last week, the first evidence of atomic
disintegration was dynamical and due solely to the
energy which is evolved in the process. The answer
to the question as to what are the ultimate products of
atomic disintegration must be looked for on quite
different lines. The ultimate products formed will be too
small for detection in the ordinary way by the statical
methods of chemistiy and physics, but they will accumu-
late indefinitely.
Since the processes go on steadily, so far as we know,
in the minerals in which the radioactive elements are
found, the ultimate products, formed through past ages
of disintegration, must accumulate therein from one
geological epoch to the next. So that at the present
day one ought to find in the radioactive minerals the
ultimate products of the disintegration process, accu-
mulated in sufficient quantity to be capable of detection
by the ordinary methods of chemistry.
Now the radioactive minerals are always very com-
plex, and contain a very large proportion of the total
number of elements known, so that in most cases it is
impossible to deduce very much from this evidence.
Nevertheless, there was one clear definite exception, and
that was the element helium. Another definite but less
unequivocal exception was the element lead.
SOLAR AND TERRESTRIAL HELIUM 97
Discovery of Helium, Solar and Terrestrial.
The history of our knowledge of helium is unsurpassed
by that of any other in interest. Its very name (from
rjX,Lo<i, the sun) stands witness to the fact that it was
known to exist in the sun as an element before it was
known to exist on the earth at all. It was discovered
in 1868 by the spectroscope in the sun's chromosphere,
by means of the characteristic bright yellow line in its
spectrum, which is technically known as " D3". Then,
in 1895, Sir William Ramsay discovered it in certain
minerals found in the earth's crust, and made a syste-
matic investigation of its physical and chemical nature.
It is a gas, the second lightest known, only twice as
dense as hydrogen, and for long was the only gas which
successfully resisted all efforts made to liquefy it by
extreme cold and pressure. In 1908, however, Kammer-
lingh Onnes succeeded by the exercise of wonderful
experimental skill and persistence in reducing helium
to the liquid state, attaining thereby a far lower tem-
perature (270° Centigrade, or only 3° from the absolute
zero of temperature) than has ever before been reached.
It is readily evolved from the minerals in which it is
found, either by heating them or by dissolving them,
but once evolved it cannot again be absorbed by the
minerals or by any other substance known. Indeed,
helium resembles argon perfectly in chemical nature,
in that it is quite without any combining power, and
exists free as single atoms without being known to form
compounds of any kind whatever. Its atomic weight
is four (hydrogen=l). Sir William Ramsay drew
attention to the fact that all the minerals in which he
found helium contained either uranium or thorium.
This was before the days of radioactivity, and for long
the origin of the helium — a non-condensable, non-
combining gas — in minerals containing uranium and
thorium was a matter for comment and speculation. In
98 HELIUM AND RADIUM
certain cases the volume of helium evolved is nearly
a hundred times as great as the volume of the mineral
in which it is contained.
Prediction of the Production of Helium.
The disintegration theory enabled Professor Ruther-
ford and myself at once to give a probable explanation
which has since proved to be correct. We regarded
helium as one of the ultimate products of the disintegra-
tion of the radioactive elements, radium, uranium, and
thorium. Forming during the long ages of the past
throughout the mass of the mineral, which is often of a
glassy nature, it is unable to escape until the mineral
is heated or dissolved, and it steadily accumulates with
the passage of geological time. We ventured to predict
that helium was one of the ultimate products of radio-
active changes, being formed in Nature from radium,
uranium, and thorium, excessively slowly, but still
fast enough to ensure that all minerals containing these
elements must contain helium also. This has since been
proved to be the case. It is true that in certain uranium
minerals — e.g., autunite and carnotite, the amount
present is often excessively minute, but these also are
just the minerals which it is believed are of extremely
recent geological formation. Indeed, the ratio between
helium and uranium or thorium in minerals is now
one of the recognised methods of estimating their age.
From this point the work proceeded along two
separate lines. Rutherford, in an exhaustive examina-
tion of the nature of the a-rays, which we have already
considered, proved first that they consisted of positively
charged atoms expelled with great velocity. At first
their mass was given as twice that of hydrogen, on the
assumption they carried one atomic charge. Then, as
the sequel to the beautiful counting experiments we have
considered, it was proved in 1908 that each a-particle
carries two atomic charges of positive electricity.
Therefore the mass of the a-particle is four, that is to
Fig. 24. — Original Spfxtrum-Tube in which the Formation
OF Helium from Radium was first observed.
Helium
Gas
from J.
Radium
Hydrogen
1 1
1 _^__^„ 1
m
Red 1
1 Violet
ir
HI
IV
Fig. 25. — Dr. Giesel's Photograph of the Spectrum of the
Gas from Radium.
II 20 minutes', III 5 minutes' exposure. I is the Spectrum of Helium,
IV that of Hydrogen for comparison.
To face p. 99
PRODUCTION OF HELIUM FROM RADIUM 09
say, it is the same as that of the atom of helium. This
made it very probable, therefore, that the a-particle
is an atom of helium.
Production of Helium from Radium.
The prediction that helium was a product of radio-
active changes was proved directly by Sir William
Ramsay and myself in 1903. We chose for the parti-
cular case of radioactive change studied that of the
emanation of radium, since it is rapid, and the emana-
tion can readily be obtained, free from other gases,
first by the action of suitable absorbents, and finally
by condensing it with liquid air and removing the gases
not condensed with a pump. So purified, it was sealed
up in a small spectrum tube, so that the spectrum of
the gas could be examined at will, and then it was left to
itself. At first no helium was present. Helium, not
being condensable by liquid air, could not have been
present in the tube as first prepared. But in the course
of three or four days, as the emanation disintegrated,
the spectrum of helium gradually made its appearance,
and finally the whole characteristic spectrum of helium
was given by the tube. Fig. 24 shows a photograph
of one of the original spectrum tubes in which the pro-
duction of helium from radium was proved. This
observation of the production of the element helium from
the radium emanation, and therefore (since the emana-
tion in turn is produced from radium) from the element
radium, has since been verified and confirmed by
numerous investigators in various parts of the world.
It has also been found by Debierne in a similar manner
by the spectroscope that actinium, a radioactive sub-
stance found by him in pitchblende, produces helium.
Dr. Giesel has actually succeeded in photographing the
spectrum of the gases generated by radium, and one of
his photographs is reproduced in Fig. 25. It represents
four separate spectra, one below the other in parallel
strips. The uppermost (I) is ordinary helium. The
100 HELIUM AND RADIUM
second and third (II and III) are two photographs
obtained from the gas generated by radium. In the
second an exposure of twenty minutes, and in the third
one of five minutes were given. The lowest spectrum
(IV) is that of hydrogen. It will be seen that many
of the helium lines are present in the spectrum of
the gas from radium. The other lines are those of
hydrogen, due, no doubt, to the presence of a trace of
moisture. The figures above and below the plate refer
to the stronger lines of helium and hydrogen respec-
tively clearly visible in^ photograph II. They refer to
the wave-lengths in Angstrom units (10 "^"^ metre).
It must be remembered that the (visually) brilliant
yellow line D3, owing to its colour, appears far less
intense in the photograph than the blue and violet lines.
Production of Helium from Uranium and
Thorium.
I was engaged for four years in an attempt to detect the
production of helium from the primary radio-elements
uranium and thorium, and succeeded in proving in
both cases that helium is produced, and, moreover,
that the rate of production is almost exactly what is to
be expected from the theory of atomic disintegration.
This quantity is about one five-hundred-thousand-
millionth of the mass of the uranium or thorium per
annum ! A photograph of the apparatus employed,
as it stood in the Physical Chemistry Laboratory, is
shown in Fig. 26. These are seven exactly similar
arrangements side by side, each of which is quite separate
and unconnected with the others. Each consists
essentially of a large flask, capable of holding a con-
siderable quantity of the material experimented upon
in the form of solution. Each is provided with a peculiar
form of mercury tap, which, while it serves perfectly
to keep out the atmosphere from the flask for an in-
definite time, can at any moment be opened by sucking
down the mercury in the barometer tubes, so that the
w :i
0. S
To face p. loo
PRODUCTION OF HELIIBI FROM URANIIBI 101
accumulated gases from the flask can be extracted and
tested for helium without admitting air. Air has been
the great trouble. A pin's-head-full of air left in the
whole of the large flask or in the solution, or leaking
in during the periods of accumulation, would completely
ruin the experiment. Most of the elaborations of the
apparatus have to do with .the preliminary thorough
removal of the air from the apparatus before the ex-
periments are commenced. The methods of testing for
helium are also entirely new. They depend on the
power I found was possessed by the metal calcium, when
heated to a very liigh temperature in a vacuum, of ab-
sorbing the last traces of all gases except the gases of
the helium and argon type. In this way the minute
amount of helium produced (usually not more than a
thousandth part of a cubic millimetre) is freed perfectly
from every other trace of gas and water vapour. Finally,
it is compressed by means of mercury into the smallest-
sized spectrum tube that can be made and its spectrum
examined. As shown in numerous special experiments,
the D3 line of the helium spectrum can be detected with
certainty if one millionth part of a cubic centimetre, or
one five-thousand-millionth part of a gram of helium
is present. Tliis is certainly the smallest quantity of
any element that has ever been detected by the spectro-
scope.
By frequently repeated experiments one can find
for each flask a period of accumulation that must be
allowed before helium can be detected in the expelled
gases, and so one can obtain a measure of the rate of
production of helium. In this way I have obtained
helium repeatedly from both uranium and thorium
salts, and the rate of production has been found to be
of the same order as that previously calculated from the
disintegration theory. For the case of uranium the
rate of production is about two milligrams of lieiium
from a thousand tons of uranium per year.
102 HELIUM AND RADIUM
Identity of the a-PARTiCLE and Helium.
The position is then this: heUum has actually been
found to be produced from the various radioactive
substances — radium, thorium, uranium, actinium —
which have in common ihe fact that they all expel
a-particles. The mass of these particles has been
measured and found to agree with the mass of the
helium atom. All a-particles have been proved to have
the same mass and to differ only in the initial velocity
of expulsion, whether expelled from radium itself,
from the emanation, from actinium, uranium, thorium,
or any other of the bodies which expel them. Hence
we are justified in concluding that the a-particle is an
atom of helium, or at least becomes one after the
velocity with which it is expelled is lost and it is brought
to comparative rest.
One further step in this long converging series of
experiments clinches the argument. We have seen
that the a-particle, though but feebly penetrating, has
a very definite small penetrating power. Now glass is
a substance that can be blown to an excessive degree of
thinness and yet retain to the full its air-tight properties.
I have succeeded in blowing small windows of glass thin
enough to allow the a-particle to get through, and yet
strong enough and tight enough to stand the pressure
of the air on one side when there was an almost perfect
vacuum on the other. So that it ought to be possible,
if the a-particle is an atom of helium, by storing the
radioactive substance in a very thin-walled air-tight
glass vessel, to get helium produced outside the vessel,
although no helium or other gas in the ordinary state
confined inside the vessel could escape. This experiment
has been performed by Rutherford and Royds with a
large quantity of radium loaned by the Austrian Govern-
ment. The emanation from the radium, which gives
a-particles and has been shown to give helium, was stored
in an excessively thin- walled but still perfectly gas-tight
RADIOACTIVE RECOIL 103
capillary tube, enclosed within a wider vessel. After
some days the gas in the outer vessel was found to con-
tain helium. It was proved that when helium was
stored in the inner tube, none got through into the outer
vessel. This final experiment clinches the proof that
the a-particle is an atom of helium.
The FmsT Change of Radium.
So we are justified in writing the first disintegration
suffered by radium:
0=0
+0
Radiutt). Emanation. Helium.
Fig. 27.
There is a great deal of evidence which proves that one
atom of a radioactive body expels but one a-particle
at each disintegration. Hence, since the atomic weight
of radium is 226, and that of helium 4, the atomic weight
of the emanation is presumably 222. This is the value
obtained by direct experiment (Chapter V.).
The above diagram is typical of no less than nineteen
different radioactive changes, in all of which an atom
of mass between 240 and 206 expels an a-particle, or
helium atom, of mass 50 or 60 times less. By the usual
dynamical law it is to be expected that the heavy
residue of the original atom, whatever it is, should
recoil in the direction opposite to that in which the
a-particle is expelled with a velocity between 50 and
60 times less than the a-particle, that is to say, with a
velocity between 150 and 250 miles a second. The
kinetic energy of this recoihng atom, since it depends
upon the mass multiplied by the square of the velocity,
will also be between 50 and 60 times less than that of
the a-particle. The velocity and kinetic energy pos-
sessed by a recoiling atom, though greatly inferior
104 HELIUM AND RADIUM
to that of an a-particle, are nevertheless greatly superior
to that possessed by an ordinary gas molecule at any
attainable temperature.
Radioactive Recoil.
The phenomenon of radioactive recoil comes into
evidence in a very curious and interesting manner,
which at the same time has proved of very great practical
utility. Very many of the products resulting from the
expulsion of a-rays, although after their formation they
are either not at all volatile or can only be volatilised
at a high temperature, yet at the moment of production
behave like volatile substances, and are carried away
under suitable circumstances from the preparation in
which they are produced, and deposited on the nearest
available surface. The best conditions are obtained by
working in a good vacuum, and charging the preparation
positively, and the surface, on which it is required to
deposit the recoil product, negatively. The residual
atom, after the a-particle is expelled, carries a positive
charge, and so is attracted to the negatively charged
surface. It is essential that the preparation should
be in the form of a very thin layer in order to give the
recoiling product a chance of escaping from it. In this
way many products, of period of life too short to allow of
their being separated by any other method, have been
isolated and identified with ease.
CHAPTER VII
THEORY OF ATOMIC DISINTEGRATION
Questions of Nomenclature.
The question, How can an element or the atom of an
element change? has given rise to many arguments, of
etymological rather than scientific importance. What
we now certainly know, and what radioactivity has
given us for the first' time the opportunity of learning
is, first, that some elements do change, and secondly,
how they change. The element radium changes, by
the loss of an atom of helium, into the efnanation, which
is about as different from radium in its chemical or
material nature as two elements well could be. The
one is a member of the group of alkaline-earth, the other
of the argon family of elements.
After all, is not this rather to be anticipated ? When
we arrange the elements in order of their atomic weights
— an arrangement which led to the recognition of what
is known as the Periodic Law (Fig. 43, p. 214) — the most
sudden and surprising differences appear between suc-
ceeding elements. Chlorine, potassium, and argon are
three succeeding elements in such an arrangement, and
there is no resemblance whatever between them. In
the nine successive transformations radium undergoes,
the atom suffers, in most but not in all, a disintegration
in which a helium atom is expelled. The heavy residues
of the original atom remaining after the successive loss
of one, two, three and so on of these helium atoms
constitute the intermediate bodies — the emanation,
radium A, radium B, and radium C — successively
produced, each from the preceding. It is therefore
105
106 THEORY OF ATOMIC DISINTEGRATION
rather to be expected that the succeeding transition-
substances produced one after the other should differ
entirely from one another in their material character-
istics. Further discoveries on this important question
are dealt with in Chapter XV.
Definition of the Atom.
Let us from the point we have gained now face
the question, which has proved a difficulty to so many,
of how it is we find that the elements and the atoms
are actually changing. The word atom is, of course,
derived from the Greek, and at first meant the indivisible
or the undivided. For a long time it had a subjective
meaning only, being the smallest particle imaginable,
rather than the smallest particle obtainable, and as
such it belongs to metaphysics, not to physical science.
The idea of the atom was first given an objective mean-
ing by Dalton. * He showed that chemical change be-
tween two elements occurs in definite proportions by
weight of the two elements. If unit weight of one is
taken, the weight of the other will have a definite fixed
value. But often the same two elements unite to form
more than one compound in different proportions.
Then, if unit weight of the one is still taken for reference
throughout, the ratio of the weights of the other in
various compounds will be simple multiples or sub-
multiples of one another, indicating that elements do
not combine in haphazard proportions, but " atom for
atom" by fixed increments or units of combination
having definite relative weight. Thus, one atom of
carbon combines with either one or two atoms of oxygen,
and for iron and oxygen the ratio is either one to one or
two to three. These units of chemical combination
of definite relative weight are the atoms of the chemist.
In all the various changes of matter which chemistry
has investigated it has sufficed to regard all combination
as taking place atom by atom, and fractions of an atom
or the subdivision of atoms has not been necessary.
THE ATOM OF THE CHEMIST 107
In compounds the component atoms preserve their
individuality and identity, because compounds can
always be decomposed to give back the same elements
out of which they are formed and not new ones. In
none of these changes does any deep change of the com-
ponent atoms themselves take place. As chemical
changes till recently were the most fundamental material
changes known, the chemist's atom fulfilled in a derived
sense the ancient meaning of the smallest particle that
exists. It did not suffer subdivision in the most funda-
mental changes known. But in this sense its meaning
was coupled with that of the particular element to which
it referred. Thus the atom of uranium is about 240
times as massive as the atom of hydrogen. An atom
of uranium is the smallest particle of uranium which
exists. An atom 240 times lighter than this is known,
but it is not uranium, it is hydrogen.
Elements and Chemical Compounds.
The discoveries in radioactivity have left this meaning
of the word atom unchanged. The atom of radium is
the smallest particle of radium that exists, and is the
unit of all the chemical changes radium undergoes.
When, by new and more fundamental changes than those
before known, it changes, it is no longer an atom of
radium. The matter formed is as unlike radium as any
body well could be. You may, if you like, regard the
radium atom as a compound of the atom of emanation,
and of the helium atom which result on its disintegration,
as it certainly is such a compound, but you must make it
quite clear that you do not mean a mere chemical com-
pound, which may at will be formed from and decom-
posed into its constituents. Were radium a chemical
compound of helium it would, as Sir William Huggins
has pointed out, show the spectrum of helium. Instead,
it shows an entirely new spectrum, clearly analogous to
but distinct from that shown by barium, its nearest
chemical relative. The spectrum of helium is not shown
108 THEORY OF ATOMIC DISINTEGRATION
until after the radium has disintegrated. The radium
spectrum does not contain a single helium line.
The most vital distinction, however, between an
element and a compound in the chemical sense is this:
both are ultimately compound. Of that there can be
now no doubt. But the energy change which attends
the resolution of an element into its constituent parts
is of an order of a million times greater than in the case
of the resolution of any chemical compound. Although
this is a question of degree, it is of a degree of so entirely
different an order of magnitude that it completely
differentiates the two types of complexes, and nothing
but confusion can result from giving to each the same
name. Radium is as much an element as any of the
other eighty. If radium is complex, so, almost certainly,
are all to greater or less degree. If radium changes,
so may (perhaps even so do) all. Their complexity is of
a completely different character from that of chemical
compounds, and it is best in the end to retain the old
words " atom " and " element " in the sense they have
had since the time of Dalton rather than attempt to
meddle with this traditional, and to scientific men,
well-understood nomenclature. The atom of the chemist
remains exactly what it was. Why, therefore, alter its
name ? If you call it a molecule, how are you to dis-
tinguish it from the chemical molecule, which has also
its own definite meaning distinct from the chemical
atom ?
The Experimental Facts.
These questions of nomenclature at first diverted
attention from the experimental fads, and gave rise
to much more or less random criticism of the younger
workers in radioactivity. Another source of con-
fusion has been the tendency to associate the discoveries
in radioactivity with other entirely distinct discoveries
made somewhat earlier with reference to the nature of
the negative electron.
It was thought at one time that it would be possible
NATURE OF ATOMIC DISINTEGRATION 109
to explain the atoms of matter as being built up entirely
of electrons or atoms of electricity, which turned out
to be as little in accord with actual evidence as it would
be to regard the solar system as composed entirely of
planets and to neglect the central sun. The problem
of the real nature of the atoms of matter has not been
completely solved by either of these independent
scientific advances.
Another objection to the validity of radioactive
evidence has been the minuteness of the amounts of
matter on which the evidence is based.
It has been stated that it is impossible to come
to any settled conclusions in regard to radioactivity,
until enough of the materials can be obtained to suffice
for the requirements of chemical investigation. But
surely, this criticism puts weight on mere familiarity
with the older methods rather than on their real in-
trinsic value. The tests by which we can recognise and
identify with ease, and measure with accuracy the
amount of, say, one billionth of a milligram of the radium
emanation, possess a philosophical foundation which
would challenge comparison with any of the tests of the
chemist on any kind of matter, in any quantity great
or small.
The Nature of Atomic Disintegration.
It is my intention to give you, so far as I am able
with accuracy, broad general mental pictures of radio-
active processes, rather than the detailed technical
investigations on which these pictures are based. Bear
in mind exactly the relation of such mental pictures to
the discovered facts. The pictures may not be true,
but they are not demonstrably false at the present time.
That is to say, you may in any case, without fear of
being led into error, apply the picture you have to what
is taking place, and the view will lead you to expect
certain consequences, and these consequences in every
known case agree with the facts. Without such mental
110 THEORY OF ATOMIC DISINTEGRATION
pictures, or generalising hypotheses, no man could
encompass even a small part of one science. So long
as the deductions from the hypothesis are in agreement
with facts and can be used to predict them accurately,
even when they are still unknown, thus saving the
memory, the hypothesis or mental picture is not even
supposed or expected to be the absolute truth. So long
as all the known facts occur as though the hypothesis
were true, the latter serves a very useful purpose,
although at any time it may be replaced by a deeper
view, one step nearer to absolute truth.
In the early history of the subject two possible alter-
natives had to be taken into account with reference to
the exact nature of radioactive changes. Radioactivity
is an atomic phenomenon, and the radio-elements are
slowly undergoing changes. What do we mean by
" slowly" in this connection ? Two possibilities arise.
Either the slow changes may result from a slow gradual
alteration, through all the atoms of a radioactive sub-
stance gradually evolving their stores of internal energy
and changing by slow degrees into new kinds of matter.
This point of view it was never possible to entertain for
a moment. Or, the change is slow and gradual with
regard only to the mass of the substance as a whole, but
sudden and explosive in character with regard to each
individual atom as its turn to disintegrate arrives.
This, from the first, the only possible point of view, is
in accordance with all that has since been discovered with
regard to the nature of the successive disintegrations
and of the a-rays expelled. Radioactive changes
proceed in cascade, from step to step, the accomplish-
ment of each successive step taking on the average
a definite time. But as regards the individual atom
disintegrating, the change is sudden in time and of
the nature of an explosive disruption, in which an
a-particle is expelled with enormous speed, and the
old atom becomes ipso facto a new one, of atomic
weight four units less. Regarding the individual
radium atom, for example, there is no gradual change
THE CHANCE OF DISINTEGRATION 111
into the emanation and helium atoms. Regarding the
whole mass of radium, there is a very gradual change
in the sense that some definite small proportion of the
whole suffers disintegration in each unit of time.
The Chance of Disintegration.
This, then, is the very vivid mental picture of atomic
disintegration which the detailed researches in radio-
activity have established. Any one radio-element like
radium being considered at any instant, among its
innumerable host of atoms, most of which are destined
to last for hundreds, some for thousands of years, a
comparatively very small proportion every second fly
apart, expelling a-particles and becoming emanation
atoms. Next second the lot falls to a fresh set to dis-
integrate, and so the process goes on, a-particles being
expelled as a continuous swarm, and yet so small a
fraction of the whole changing that the main part of
the radium will remain unchanged even after hundreds
of years. Now consider the emanation atoms formed.
These are much less stable than the atoms of radium.
A much larger fraction of these disintegrate every second,
producing more a-particles and a new body not yet
considered.
It is now necessary to consider briefly the exact
nature of radioactive change and the laws it follows.
The deduction of these laws is a matter for the mathe-
matician. We are chiefly concerned with the general
conclusions which have transpired. I will first state
the most important of these in words divested of mathe-
matical symbols. The chance at any instant whether
any atom disintegrates or not in any particular second
is fixed. It has nothing to do with any external or
internal consideration we know of, and in particular
it is not increased by the fact that the atom has already
survived any period of past time. The events of the
past in radioactive change have, so far as we can tell,
no influence whatever on the progress of events in the
future. This follows from the consideration of the one
112 THEORY OF ATOMIC DISINTEGRATION
general mathematical law which all known cases of
atomic disintegration so far investigated have been
found to follow. Fortunately the law itself is simple. Its
application in individual cases is often complicated, but
I shall confine myself to the simplest, which are at the
same time the most generally important, consequences.
The chemist has to do with many types of change all
following different laws. In some the rate of change —
that is, the quantity of the substance changing in the
unit of time — is proportional to the quantity present of
the substance which is changing, in others to some power
of this quantity. Now, in radioactive change the rate
of change is invariably simply proportional to the quan-
tity of changing substance. This seems easy enough,
but I would warn the uninitiated that they must not
overlook the important fact that since the quantity of
a changing substance itself changes as time goes on,
owing to the progress of the change, the rate of change
being proportional to the quantity also continuously
changes, and at no time has a constant value. Hence
you cannot get much further by simple arithmetic and
algebra. Of course, in the case of a slow change like
that of radium itself, when even in a lifetime the quan-
tity of radium is not very appreciably reduced by the
operation of the change, it is allowable to neglect the
slow alteration of the rate of change with the time and
to consider the rate of change as constant, since for short
periods of time it essentially is so. In most cases some
knowledge, withal a slight one, of the mathematics of
continuously varying quantities is essential for the
complete deduction of the laws of radioactive change.
However, as my intention is to avoid mathematics,
I shall simply state these consequences ex cathedra.
The Period of Average Life of a
Disintegrating Atom.
The rate of change in any single case of atomic
disintegration is proportional to the quantity of the
substance which is changing. The usual plan is to let
THE RADIOACTIVE CONSTANT 113
the symbol \ represent the fraction of the total changing
per second, and to this symbol X is given the special
name " the radioactive constant." X may represent
a small or a large fraction, according to the particular
case, according as the disintegration process is slow
or rapid. The important point is that it is a real con-
stant of nature in every case, independent of the past
and future history of the substance, its actual amount
whether large or small, and of every other consideration
whatever. Thus for the emanation of radium, X, the
radioactive constant, has the value 1/481,250, which
signifies that in this case 1/481, 250th of the total amount
of emanation in existence changes per second. The next
step, skipping the mathematics,-' is that the average period
of life of the atom of a radioactive substance — that is
to say, the period of time in seconds it exists on the
average before its turn comes to disintegrate — is simply
the reciprocal of the radioactive constant, or 1/X.
Thus the average life of the radium emanation is 481,250
seconds, or 5-57 days.
Now as radioactive change proceeds during every
instant at the rate proportional only to the total quan-
tity of substance undergoing the change, which is
present and remains unchanged at that instant, and as
in this method of looking at the changes we do not
consider at all the absolute quantities, only the fraction
of the whole changing, it follows that X is always of the
same value throughout the process from start to finish.
It also follows that l/X, the period of average life of
the remaining atoms, does not, as you might be inclined
to suppose, tend to lessen as time goes on. The atoms
disintegrating first have a far shorter period of life, and
those disintegrating last have a far longer total period
than the average. But at any instant throughout, con-
sidering only the atoms still remaining unchanged at
that instant, then from that instant the average period
of life is always l/X.
1 So far as I know, the period of average life was first deduced by
Mr. J. K. H Inglis, to whom I put the problem.
114 THEORY OF ATOMIC DISINTEGRATION
Our own period of average life, of course, follows
very different and far more complicated laws. The
expectation of life at any age is a practical problem for
the actuary. But every one knows, owing to the mor-
tality among infants, that the expectation of life at
birth is less than shortly afterwards, when it reaches a
maximum and then gets less and less with increasing age.
The " expectation of life " of a radioactive atom is
independent of its age — as it happens the simplest
possible law and one lending itself, as will appear, to
some most beautiful deductions. That this is so can
be directly proved in the simplest way, by comparing,
for example, the rate of change of newly-born radium
emanation, not in existence a few minutes before, with
that of the residue of an originally much larger quantity
that has survived a period several times greater than the
period of average life.
The Unknown Cause of Disintegration.
This answers fully the general question. How does
an element change ? You will probably wish to know
why it changes in this particular way. That cannot
be said, although the true answer would undoubtedly
take us far. All that can be stated is that the immediate
cause of atomic disintegration appears to be due to
chance. If the destroying angel selected out of all
those alive on the world a fixed proportion to die every
minute, independently of their age, whether young or
old, if he regarded nothing but the number of victims
and chose purely at random one here, and one there, to
make up the required number, then our expectation
of life would be that of the radioactive atoms. This,
of course, is all that is meant by the statement that the
course of atomic disintegration appears to be due to
the operation of " chance."
It is natural to inquire why this particular law is
followed. On this fundamental question no light is
yet forthcoming. There is always " a cause of the
THE PERIOD OF HALF-CHANGE 115
ultimate cause." Atomic disintegration is assuredly
the ultimate cause of radioactivity. It does not
weaken this deduction that as yet we have not found
the ultimate cause of atomic disintegration. Various
possible causes have been discussed. Most of them,
so far from helping the elucidation of the " why," do
not conform even to the " how." The law of radio-
active changes shows clearly that the past history of an
atom does not increase its chances of undergoing dis-
integration in the future, which is a fundamental step
gained, although it leaves the ultimate problem unsolved.
There is another way of stating the law of radio-
active changes, and that is by saying that as the time
increases in arithmetical progression the amount of
substance remaining decreases in geometrical progres-
sion. Suppose in a time of r seconds one-half of the
total amount changes and one-half remains unchanged.
In the next period of t seconds, 2 r altogether, one-half
of what is left — that is, one-quarter — changes, and one-
quarter of the total remains unchanged. In 2 r the
quantity is reduced to 1/2^. In any period of time
represented by N r seconds, where N is any multiple or
submultiple, the quantity of substance remaining is
1/2". It remains to state what relation the time r
required for the half-change to occur, bears to the period
of average life l/X of the former way of considering the
change. There is a fixed ratio between these two
periods, the latter being always 1-45 times the former.
In a time equal to the period of average life l/X, the
quantity of substance present is reduced to l/e=0-368
of the initial quantity.
Determination of the Period of Average Life.
These considerations would have little interest to
us but for the fact that they afford the means whereby
the period of average life of any radioactive element can
by their aid be exactly determined, not only for those
transition-bodies like the emanation, which change so
116 THEORY OF ATOMIC DISINTEGRATION
rapidly that we can watch their complete transformation
in the course of a few days or weeks, but also for the
primary radio-elements, some of which we know require
thousands of millions of years to run their course of
change. The average life of a radioactive element,
representing as it does a fundamental constant of nature,
is one of its most important attributes. Our own
period of average life being strictly limited, it naturally
affects very much our way of looking at the various
radioactive bodies. If, for example, the average life
is a matter of a few days, as in the case of the radium
emanation, we regard the body as an ephemeral transi-
tion-form. If it is, as in the case of radium, a few
thousand years we are inclined to look upon the sub-
stance as a permanent and primary radio-element.
There is really not this sharp difference. But it is con-
venient to divide radioactive bodies into two classes,
and in the one to put those for which the periods of
average life are short compared to our own, and in
the other to put those for which the periods are long.
The method employed to determine the value of this
fundamental and all-important constant is naturally
quite different in the two cases. In the first, simple
direct observation suffices. Thus if we measure the
decay of the activity of any separated quantity of the
emanation of radium with time, we shall find that it
decays in a geometrical progression with the time to
half its initial value in the course of 3-84 days. The
period of average life is 1-45 times greater, or 5-57 days.
But in the case of a body, of which one thousandth, or
one thousand-millionth as the case may be, changes
annually, simple direct observation does not help much.
How are we to proceed ?
In the first place, let us consider the cases of uranium
and radium. We may determine how many times
more powerfully radioactive radium is than uranium.
The radioactivity of radium is several million times that
of uranium when the a-rays of equal quantities of the
two elements are compared. From this it may be con-
THE PERIOD OF AVERAGE LIFE 117
eluded that the period of uranium is several million times
longer than that of radium, and if the latter is known,
that of uranium may be roughly estimated, although
it is a period of some thousands of millions of years.
As a matter of fact, there is a very beautiful generalisa-
tion, I have already referred to briefly, and which
later on I shall try to develop further by the aid of an
analogy, by means of which the periods of average life
of the radio-elements of the second class, those, that is,
which are long-lived compared with ourselves, have
come into the region of exactly knowable quantities.
If the period of average life of a single member of a
series of successive atomic disintegrations is knoAvn
the others can be calculated, provided certain data, not
entirely impossible to obtain, are known. It will clear
the ground considerably if I attempt to give you the
main idea succinctly in the case of radium itself and of
the first product of its disintegration, the emanation of
radium. I have already alluded to the fact that owing
to the very rapid disintegration of the emanation its
quantity does not continuously accumulate, but reaches
an equilibrium ratio with respect to the radium pro-
ducing it, in which the amount of already formed
emanation disappearing is exactly counterbalanced
by the amount of new emanation formed.
The Period of Average Life of Radium.
This state of things is known generally by the name
of radioactive equilibrium. The importance of the
existence of this state of radioactive equilibrium it is
impossible to overrate. Many problems, as we shall
come to see, which, to us with our limited period of life,
might well appear absolutely insoluble, connected as
they are with periods of time so vast that our little
life by comparison appears a mere moment, are solved
directly by the proper application of this principle. Now
I am only giving you the main idea and one specific
illustration of what is in fact a law of great generality.
118 THEORY OF ATOMIC DISINTEGRATION
, By the law of radioactive change, if X^ is the radio-
active constant of radium — i.e., the fraction of the
whole changing per second — and N is the total number
of radium atoms dealt with, then the number of radium
atoms changing into the emanation per second, and
therefore also the number of atoms of fresh emanation
produced per second, is XiN. But in equilibrium this
equals the number of emanation atoms disappearing.
If the radioactive constant of the emanation is X2, and
the number of atoms of emanation present during
equilibrium is denoted by X, the number of emanation
atoms disappearing per second is X2X. Hence we have
\,N=\n'K and i:T=r^«
^ ^ N X2
This law, the most important in radioactivity, thus
states that in successive disintegrations the product
accumulates in quantity until a fixed ratio with respect
to the parent body is attained, and this ratio is inversely
proportional to their respective radioactive constants
or directly proportional to their respective average lives.
It is necessary for the law to hold true that the period
of the parent body should be much longer than the
periods of any of its products, and in this case the product
selected need not necessarily be the first product, but
may be any one of the successive products formed in
the series.
X2 is well known by direct observation. Now if
X/N, the ratio between the number of atoms of emana-
tion and of radium in equilibrium together, can be
found, then \i, the radioactive constant and therefore
1/Xi, the period of average life of radium can be de-
duced. That is the important thing — the period of the
average life of radium, the rate at which it is changing,
and a host of vitally important consequences, can be
deduced. For a slowly changing body like radium
the second is an inconveniently short unit of time to
employ, and it is better to take a year. What is wanted
is the fraction of any quantity of radium which changes
AVERAGE LIFE OF RADIUM 119
in a year. The quantity X/N, which is the ratio of
the number of atoms of emanation and of radium in
equiUbrium together, can be deduced by ordinary
physico-chemical laws if the actual volume of emanation
in equilibrium with a given quantity of radium can
be determined. As already mentioned (p. 82), this
volume was first approximately measured by Sir William
Ramsay and myself in 1904. The actual volume of
emanation is excessively minute, but it is just within
the range of measurement. From our results we con-
cluded about l/l 150th part of the radium changes
annually, so that the period of average life on this
estimate is 1,150 years. Owing to the excessive minute-
ness of the volume, the method is not an accurate one,
tending, since the volume of emanation is likely to
be too great unless every trace of other gas is absent,
to give too short a period. Later experiments by the
same method with much larger quantities of radium
have shown that the correct value is about double that
first found. With the gro^vth of the subject other
methods, less direct but more accurate, have become
available. Professor Rutherford recently, from a con-
sideration of a large number of separate data
accumulated by himself and others bearing on this
question, came to the conclusion that the period of
average life of radium is not very far removed from
2,500 years, and we shall take this value as the most
probable. It may suffer slight further alteration as
fresh data are accumulated, but it is very improbable
that it is seriously in error. Within narrow limits
the average life of radium may be taken to be 2,500
years.
The Total Energy evolved in the Complete
Disintegration of Radium.
A knowledge of this important constant enables
us at once to say how much energy any quantity of
radium would evolve in the course of its complete
120 THEORY OF ATOMIC DISINTEGRATION
change — that is, during a period of some thousands of
years. We saw (p. 22) that a gram of pure radium
evolved about 133 calories of heat per hour. There are
8,760 hours in the year, so that in a year a gram of
radium evolves about 1,160,000 calories. In a year
1 2500th part changes. Therefore in the complete
change of one gram of radium no less than 2,900,000,000
calories would be evolved. The energy evolved in the
change of radium is nearly a million times greater than
that evolved from a similar weight of matter undergoing
any change known previously to the discovery of radio-
activity. By the burning of a gram of coal, for example,
only about 8,000 calories are obtained. In this change,
however, 2| grams of oxygen are also consumed, so
that per gram of the two substances taken together the
heat evolved is only 2,200 calories. On this basis
of calculation the energy of radium is well over a million
times that furnished from the combustion of coal.
No wonder then that to account for the boundless energy
displayed everywhere in the starry heavens proved a
difficult problem for physicists, acquainted with no
more energetic chemical process than the burning of
coal !
CHAPTER VIII
THE ORIGIN OF RADIUM
Why has Radium Survived ?
One of our chief duties will be to follow out this theory
of the disintegration of atoms in radioactivity. The
bare idea of elements spontaneously changing raises so
many obvious and apparently insurmountable difficulties
that it will be interesting to consider them as they arise
and to consider what answer can be made to them.
To-night we must concentrate on one of the chief of
these — a difficulty which no doubt has already pre-
sented itself in many of your minds. If radium is
changing at the rate of nearly one two-thousandth
part every year, how is it that there is any radium
left at the present time ? Even at the beginning of
the time recorded in past history there must have
existed several times as much radium as there is now,
if the rate of disintegration has been constant over that
period, while a hundred thousand years ago it can be
calculated that there must have existed a thousand
billion times as much as to-day, had the steady disin-
tegration been going on at its present rate. That is to
say, even if the whole world were originally pure radium,
in a period of time brief compared to that which we
know from geological evidence it has actually been in
existence, there would be practically none left, and
certainly not as much as actually exists to-day. Or,
looking forward instead of backward, if we put this half-
grain of radium bromide in a safe place, and then could
revisit the earth say twenty-five thousand years hence,
we should find less than one-thousandth part of it
121
122 THE ORIGIN OF RADIUM
remaining. The slow disintegration would have done its
work and changed the radium into the non-radioactive
elements which are being formed from it. This question,
apparently so insoluble, in reality admits of the most
direct and satisfactory answer on the disintegration
theory and serves as a good example of how a theory, if
it is worth the name, must be able to predict future
discovery as well as to explain the existing facts.
An analogy to facts we have already discussed will
help us to find the solution of this difficulty. In the
emanation of radium we have become acquainted with a
body changing so rapidly that at the end of a month none
of the original quantity remains. How is it there is
any emanation in existence at all ? Because it is being
reproduced as fast as it disappears. Is there any
reproduction of radium going on, balancing the effect
of its disintegration and maintaining its quantity from
age to age ? Radium is the direct parent of the emana-
tion. Itself changing more than a hundred thousand
times slower than its product, it maintains the quantity
of emanation in existence over a period a hundred
thousand times longer than would otherwise be the
case. Is there then a parent of radium ? Does there
exist any other element producing radium by its own
disintegration as fast as that already in existence
disappears ?
The Reproduction of Radium.
Do not regard this thirty milligrams of radium
bromide as something merely by itself. Consider its
history. By infinite labour and patience this tiny
quantity of radium has been separated from several
hundredweights of the mineral pitchblende. Suppose
in this operation all the rest of the mineral, after the
extraction of the radium, were preserved and put in a
safe place. When we revisited our specimen of radium
twenty-five thousand years hence, and found practically
none of it remaining, should we find that the mineral
from which it was extracted had in the meantime grown
REPRODUCTION OF RADIUM 123
a fresh crop of radium ? The answer is that we
should.
This was one of the first predictions made from the
theory of atomic disintegration and one of the most
recent to be confirmed by experiment. Long before
the data were available which enabled an exact estimate
of the life of radium to be calculated, it was recognised
that radium, though at first sight a permanent and
primary radio-element, is changing so rapidly that, had.
there existed no process in which fresh radium is supplied
to replace that changing, none could possibly have
survived till the present day, and from general principles
it was possible to make a shrewd prediction as to which
element was the parent of radium. We have already
considered the general principles which enabled the
prediction that helium was one of the ultimate products
of radioactive changes to be made. Ultimate products
must co-exist with the radio-elements producing them
in all the natural minerals in which the latter are found.
Something of the same reasoning applies to the parent
of radium, only in this case it is far more definite and
elegant. The parent of radium must co-exist with
radium in all minerals in which radium is present. Now
it is at once obvious, if this explanation of the parent of
radium is to meet the case, that such a body must be
changing very much more slowly than radium, otherwise
there would arise the same necessity to assume the
existence of a parent of the parent as there is of a parent
of radium. The original first parent of radium must be
changing excessively slowly to maintain a steady supply
of radium over long epochs of geological time.
The Ratio between the Quantities of Uranium
AND Radium in All Minerals.
By the law already formulated on p. 118, in two succes-
sive, not necessarily consecutive, disintegrations of which
the second is much more rapid than the first, the more
rapidly changing body accumulates in quantity until
a fixed ratio with respect to the parent body is attained,
124 THE ORIGIN OF RADIUM
and this ratio is inversely proportional to the ratio of
their respective rates of change, or directly proportional
to the ratio of their respective periods of average life.
Let us apply this law. The parent body is the parent
of radium. The quantity of radium in minerals must
therefore attain a fixed ratio with respect to the quantity
of the parent of radium, and this ratio is the ratio of
the period of average life of radium to that of its parent.
The quantity of helium that accumulates in a mineral
continually increases as time goes on, assuming the
helium does not succeed in escaping, and no definite
proportion between helium and radium is to be ex-
pected. But the case is different with radium and its
parent. There must be a fixed ratio, independent of
the age of the mineral examined. As the original
first parent of radium must be changing excessively
slowly to survive geological epochs of past time, there
must be always a very large quantity of it in the mineral.
As the radium is changing, from the standpoint of
geological epochs of time, very rapidly, there must
always be a very small quantity of radium. Between
these quantities great and small there must exist the
same ratio as between the respective periods of average
life of the two bodies.
A very cursory examination of the minerals in which
Mme. Curie found radium was sufficient to point strongly
to the probability that uranium is the primary parent
of radium. Uranium was, as we have seen, the original
element for which the property of radioactivity was
discovered, and its radioactivity is several million times
more feeble than that of radium. Now the radioactivity
depends only on the atoms actually breaking up, and
therefore in comparing uranium with radium it follows
that uranium must be disintegrating several million
times more slowly even than radium, so that if uranium
produces radium the quantity of uranium must be
several million times greater than the quantity of
radium in minerals. But this is exactly what Mme.
Curie found to be the case in the minerals she worked
QUANTITY OF RADIUM IN MINERALS 125
up for radium. So that from the very first there existed
a strong presumption that uranium is the original parent
of radium. The evidence in support of this view at
the present time is complete and satisfactory. We owe
it to the careful work of McCoy, Strutt and Boltwood
that the genetic relation between uranium and radium
has been established. They determined the ratio
between the quantities of uranium and of radium in a
large number of minerals. In every mineral examined
containing uranium there was found to exist a direct
proportionality between the quantity of uranium and
that of radium. To Rutherford and Boltwood together
we owe the exact determinations of this important con-
stant of proportionality. They found that for every
one part of radium there always exists 3,200,000 parts
of uranium. This constant gives directly, unless other
undetermined factors interfere, the ratio of the average
lives of the two elements. As we have seen, that of
radiurji is 2,500 years. Hence it follows that that of
uramum is 8,000,000,000 years. Enormous as this
period is, it is not now merely a deduced or calculated
value. I obtained the same result by direct experiment
from the rate of production of helium from uranium.
Hydraulic Analogy to Radioactive Change.
It will help us considerably if we try to find some
analogy to the important and intricate relations that
exist between uranium and radium. We may take for
illustration the magnificent system of waterworks which
supply this city, which we will suppose have been given
over to us by the Corporation to control for the purposes
of our illustration. As you know, we in Glasgow are
supplied ultimately from Loch Katrine through an
intermediate reservoir at Milngavie. We shall first cut
off Loch Katrine from all fresh sources of supply of
water, and from all outlets except to the intermediate
reservoir at Milngavie, and we shall see to it also that
the latter receives no water except from Loch Katrine,
10
126 THE ORIGIN OF RADIUM
and delivers none except to Glasgow. We shall then
issue to our engineers the instructions that there must be
delivered every hour at Milngavie from Loch Katrine
approximately one eight-millionth part of the total
store of water in Loch Katrine, and from Milngavie to
Glasgow every hour one two-thousand-five-hundredth
part of the total store of water at Milngavie. Then, if
instead of hours we read years, the quantity of water
in Loch Katrine represents the quantity of uranium,
and the quantity of water in Milngavie that of radium.
For the sake of brevity we shall term Loch Katrine the
source and Milngavie the reservoir.
First we shall suppose that our regulations have
been in operation already a considerable number of
hours, as this is the condition in which, reading years
for hours, we find uranium and radium together in
minerals in Nature, for example, in a piece of pitchblende.
What relation will the quantity of water in the source
bear to the quantity in the reservoir — that is, the quan-
tity of uranium to the quantity of radium ? The amount
of water the reservoir receives is quite independent of
the amount it contains, but the amount it delivers is
proportional to the amount it contains. Similarly the
amount of radium produced from uranium does not
depend at all on the amount of radium already present,
while the amount that itself changes depends only
on and is proportional to the amount present. Never-
theless, we shall find that there is about three million
times more water in the source than in the reservoir.
Because only under this condition is the intake of the
reservoir equal to the outflow from the reservoir — that
is, the production of new radium equal to the disappear-
ance of the old. Imagine, for example, that there was
just twice as much water as this ratio in the reservoir,
then twice as much would flow out as flows in, and the
supply in the reservoir would be rapidly depleted. Or,
if there were but one half as much in the reservoir,
twice as much would flow in as out, and the supply in
the reservoir would increase. In either case, intake
THE AGE OF PITCHBLENDE 127
and outflow would ultimately become equal, and no
further change would then occur until both the source
and the reservoir were empty. But let us now dis-
connect Loch Katrine from Milngavie reservoir, which is
equivalent to separating, as Mme. Curie did, the radium
from the uranium in pitchblende. Obviously the
reservoir by itself will now be able to supply water for
a very much shorter time than it did before, and, in
general, with the conditions stated, source and reservoir
together will last three million times longer than the
reservoir alone. The radium on the table will have half
disintegrated, so that only half will remain, in about
1,700 years. Whereas had it remained in the mineral
associated with its parent uranium, the quantity of
radium in the mineral will not be reduced to one half
what it is now until 5,000,000,000 years have elapsed.
The Age of Pitchblende.
Thus we can say, following a cautious reservation
once made by Professor Tait, provided the causes
that are now at work have always been in continuous
operation in the past as they are now, and that we know
of all the causes that have been at work, 5,000,000,000
years ago there must have been about twice as much
uranium and radium in this piece of pitchblende as
there is to-night. Since, however, there is actually
in this pitchblende now over 50 per cent, of uranium, it
is not possible that it can have been in existence in its
present form more than 5,000,000,000 years. But,
even from a geological point of view, this is a very long
period of time indeed; longer, perhaps, than it would be
profitable in the present state of science to push back
our inquiries. That, then, is the position with regard
to the maintenance of radium in Nature. Even when
we deliberately leave out of account the possibility there
may exist in Nature entirely unknown processes re-
plenishing the supplies of uranium, just as there are
replenishing Loch Katrine, there is no difficulty in
128 THE ORIGIN OF RADIUM
accounting for the continuous maintenance of radium
over a period of the past as great as, or greater than,
there is any reason to beUeve the earth has been in
existence in its present condition. This is as far as we
need pursue our analogy for the moment, but we shall
again find it useful at a later period. We must pass
on to another aspect of the question.
Uranium X.
At this stage it will be well to make a short digression
into the radioactivity of uranium itself, and how it
is explained on the theory of atomic disintegration.
Uranium and its compounds in their normal state give
out both a- and /S-rays. As in all other cases, the yS-rays,
being photographically the most active and being the
more penetrating, were the first chiefly studied. Sir
William Crookes and also M. Becquerel found that by
certain chemical processes a new substance in minute
quantity could be separated from uranium, to which
Crookes gave the name uranium X, and this new body
produced the whole of the photographic activity of
uranium. The uranium after this treatment no longer
affected a photographic plate. Crookes concluded that
the radioactivity was due in reality to the presence
of the foreign substance in minute amount, which he
called uranium X, and that pure uranium was not
radioactive. I repeated these experiments, and found
that only the y8-rays of uranium belonged to the uranium
X. Uranium freed from uranium X gave its normal
amount of a-rays. Then it was found that the y3-radia-
tion of uranium X decayed steadily in a geometrical
progression with the time, whereas the uranium that
had been freed from uranium X and at first gave no
/S-rays, gradually and completely recovered its power
of producing /3-rays. Uranium grows uranium X, in
exactly the same way as radium grows the emanation.
The activity of uranium X after separation from uranium,
consisting entirely of /3-rays, steadily decays in a geo-
URANIUM X 129
metrical progression with the time, faUing to one half
the initial value in 24-6 days. The average life of
uranium X is thus 35-5 days.
The disintegration of uranium up to the point so far
discussed is represented on the following scheme:
^ ( 234 J — )
Uranium. Uranium X.
8,000,000,000 years. S5-5 days.
Fig. 28.
This is as far as the methods of radioactivity enable
us directly to trace the disintegration of uranium at
the present time. Thp substance produced — uranium
X — is only an ephemeral transition-form, lasting on the
average 35-5 days, and when it disintegrates, the process
appears to come to a stop so far as our experimental
methods have yet been able to disclose.
Now, on the view that has been developed that
uranium is the parent of radium, it is natural to suppose
that uranium X in the course of time turns into radium.
A little consideration will show that if this were the case
it might easily be overlooked at first on account of the
very long period of life of radium compared with that of
uranium X. As already explained (p. 92), chemical
and spectroscopic methods of detecting matter depend
only on quantity, but radioactive methods depend upon
quantity divided by life. Assuming equal effects
produced in the disintegration of an atom of uranium X
and of an atom of radium, since the life of the latter is
30,000 times that of the former, it will be necessary
to have 30,000 times as much radium as of uranium X
to produce equal radioactive effects.
Attempts to detect the Growth of Radium.
In 1903 I started a series of special experiments
which have been continued ever since, partly in con-
130 THE ORIGIN OF RADIUM
junction with Mr. T. D. Mackenzie and more lately with
Miss A. F. Hitchins, to see whether uranium does, in
fact, produce radium. The uranium, after being puri-
fied as completely as possible by chemical methods from
radium, is left sealed up in a flask and is periodically
tested to see if a growth of radium has occurred. The
method of testing for minute traces of radium is a very
simple and accurate one, allowing quantities of radium
of only a few million-millionths part of a gram to be
detected with certainty and measured with exactitude.
Use is made of the characteristic emanation generated
by radium. Uranium does not generate any emana-
tion. The uranium solution to be tested for radium,
after standing sealed up in a glass flask for a period of
at least a month to allow the equilibrium quantity of
emanation to accumulate, is boiled in a vacuum, and
the gases expelled are collected and introduced into a
sensitive gold-leaf electroscope. If radium is present
in the solution, its emanation causes the leaf to lose
its charge, and the rate at which the discharge occurs
under defined conditions can be used accurately as
a measure of the amount of radium present. The
test is qualitative as well as quantitative, and there
is no possibility of making a mistake as to the identity
of the emanation and of the radium from which it is
formed.
The first result of these experiments, while they
furnished the first evidence of a growth of radium,
withal in very minute amount, showed that this growth
is not due to uranium. In the first experiments the
uranium salt was only specially purified from radium,
not from any other impurities that might have been
present, derived from the minerals from which uranium
is obtained, and a very slow growth of radium from the
preparation was actually observed.
In later experiments more perfect methods of purify-
ing the uranium initially were adopted, with the result
that the growth now of radium occurred chiefly in the
impurities separated, whilst the growth in the purified
FIRST EXPERIMENTS 131
radium was reduced to an excessively minute amount.
In these the greatest growth recorded was only one fifty-
millionth of a milligram of radium after six years. At
this rate, even at the present enormous price of radium,
it would require sixty thousand years to produce one
pennyworth.
Now if uranium X, when it disintegrates, produced
radium directly, then with the quantities of materials
used in these later experiments, the amount formed in a
single hour would be greater than has actually been
formed in six years. In the earlier experiments, with
not specially purified uranium, the growth of radium,
although quite detectable, was still only one thousandth
part of what would have occurred had uranium X
changed directly into radium. In spite of this appar-
ently conclusive negative result, it was practically certain
that uranium is the original parent of radium, and that
in the course of years our preparations would begin to
grow radium.
Existence of Intermediate Products.
The natural explanation of this failure to detect a
growth of radium from uranium is, that one or more
intermediate bodies of long life exist in the disintegration
series between uranium and radium. On the analogy
proposed, this means that between Loch Katrine and
Milngavie reservoir one or more large intermediate
reservoirs exist, which have to fill up before the water
reaches Milngavie. Uranium X represents the first of
such a series of intermediate reservoirs, it is true, but
owing to its short period of life and the large fraction of
the total quantity always passing through on the way to
the next, such a reservoir would be an extremely small
one, and for periods such as we are considering its effect
on the flow would be practically negligible.
It would be quite otherwise if one or more reservoirs
as large as Milngavie — if one or more intermediate
substances as long-lived as radium — existed in the series.
132 THE ORIGIN OF RADIUM
1 well remember one fact told me by the engineer in
charge of the magnificent scheme of waterworks, supply-
ing the mines at Kalgurli, in Western Australia, from
a source near the coast across three hundred miles of
desert. There are several intermediate reservoirs on
the way. The plant installed is capable of pumping
five million gallons of water daily, and yet it took
a period of many weeks since pumping operations began
before the water appeared in Kalgurli. When uranium
is carefully purified from all other substances one can be
sure that one starts with all the intermediate reservoirs
empty — ^that is, with none of the intermediate substances
present. Water is flowing steadily from the source all
the time, as the disintegration of uranium is always
going on. We watched and waited seven years at the
radium reservoir — strictly speaking, at the one beyond
radium, since the emanation of radium, not radium
itself, is actually employed for the test. But the flow
had not reached there yet and the radium reservoir
remained practically as empty as at the start. But
there was no doubt it would come, and there was good
reason to expect that some of us, at least, would be still
alive when it arrived.
It is not beyond the resources of mathematics to
find out a good deal about these intermediate reservoirs.
The present results indicate that if there is but one long-
lived intermediate body between uranium and radium,
then its period of average life must be at least 100,000
years, that is, forty times that of radium itself. Also,
that the radium, in this case, must be produced at a
rate proportional to the square of the time from purifi-
cation, the growth in a century being a hundred times
as great as that in the first decade. On our analogy,
then, between Loch Katrine and Milngavie, there must
exist a reservoir of forty times the capacity of Milngavie,
provided there is only one. Since the equilibrium
quantity to which an intermediate body accumulates is
proportional to its period of average life, then if there is
only one intermediate parent of radium between radium
IONIUM 133
and uranium, there must be forty times as much of it
in minerals containing radium as there is of radium
itself.
Ionium.
This leads me to the next step. The failure to detect
a production of radium from uranium merely fore-
shadowed the actual discovery of an intermediate sub-
stance of long period of life. Boltwood in America suc-
ceeded in isolating it from minerals containing radium,
and it proves to be the direct parent of radium. It
possesses the property of producing radium directly
from itself by disintegration, and it has been called
ionium. It expels a-rays during its disintegration into
radium, and these a-rays possess a relatively low velocity.
Their range is very little more than one inch of air.
Chemically, ionium resembles thorium so completely
that the two substances, if mixed, cannot be separated.
This gives the means of separating the new body from
minerals. Some thorium is added and separated by the
well-known methods of chemical analysis. It is then
purified as completely as possible. The parent of
radium is not separated from the thorium by this treat-
ment, although all other substances are. The chemical
resemblance between these two different elements is
complete. Later we shall come to recognise many other
cases of the same kind. Ionium and thorium are what
are now called isotopes.
The disintegration series thus reads:
^ <0'* y^'^y^ p*" P" P"
I 238 )-— ^ I 23* )~-^ — — — **^( 2^° j"""^ { ^2^ 1 ■/ I 222 I— —
Uriniuml. Uranium X. Ionium. Radium. Emanation.
8,00,000,000 35-5 days. 100,000 years. 2,500 years. 5-6 days.
years.
Fig. 29.
as far as we have yet considered it. In the centre is
placed the known or presumed atomic weights of the
various bodies.
134 THE ORIGIN OF RADIUM
Production of Radium by Uranium.
Going back to the purified uranium preparations, in
1915 with the help of Miss Hitchins, the measurements
of the quantity of radium present first clearly estab-
lished that there was a steady growth of radium, and,
moreover, that it was proceeding proportionally to the
square of the time, as the theory requires. This growth
has continued regularly up to the present time (1919).
The period of average life of ionium calculated from it is
almost exactly 100,000 years. The amount of radium
in some of the preparations is (1919) about ten times as
great as initially. But the problem has taxed to the
uttermost even the extraordinarily delicate tests for
radium. For the preparation containing the largest
quantity of uranium — namely, three kilograms calcu-
lated as the element — the growth of radium after ten
years has been only one five millionth of a milligram —
i.e., one part of radium from fifteen billion of uranium.
The Stately Procession of Element Evolution.
So far, then, as we have inquired, uranium, uranium
X, ionium, radium, and the emanation represent
respectively the starting-point and the four successive
stopping-stations in the long journey of continuous
devolution from the heaviest and most complex atom
known into less heavy and complex atoms which is going
on around us, or, to preserve our original analogy, the
source and four successive intermediate reservoirs in
the flow of elementary evolution. " AH things flow "
was one of the dogmas of ancient philosophy, and in this,
as in many others, the ancients guessed truer than they
knew. Instead of four stopping-stations or inter-
mediate reservoirs in this stately procession of elements
disclosed by radioactivity, there are now known no
less than thirteen, starting from the element uranium,
but for our present purposes of illustration these four
will suffice. But this new transformation scene on
ELEMENTARY EVOLUTION 135
which the curtain of the twentieth century has been
rung up, beginning as it has done with the transforma-
tion of the most fundamental and permanent of the
existences which physical science has recognised in
the past, extends beyond physical science and trans-
figures with new light some of the most fundamental
and permanent ideas which in one form or another are
deep-rooted in the world's philosophies.
CHAPTER IX
THE SUCCESSIVE CHANGES OF RADIUM
The Later Changes of Radium.
We have attempted to trace radium to its source.
It remains to follow through its disintegration briefly
to the end. This was a task to which Rutherford
particularly devoted himself, after the main principles
of atomic disintegration had become famiUar, with the
consequence that, with the exception of a lacuna here
and there still to be supplied, our knowledge of the whole
process from the start to finish is now tolerably complete.
In addition, some new considerations have transpired
which concern us nearly in the broad general application
of the principles of atomic disintegration, so that for this
reason, if for no other, the work claims our attention.
Most of you who have read at all in the subject will
be aware of one mysterious and extraordinary power
possessed by radium, which I have hitherto carefully
avoided all mention of, not wanting to have too many
irons in the fire at once. Radium possesses the power
of endowing with some of its own radioactivity neigh-
bouring objects. Thorium, which is very like radium
in many ways, particularly in giving a gaseous emanation
(which, however, has the very short period of average
life of only a little over a minute), also possesses a
similar power. The phenomenon was discovered by the
Curies for radium and termed " induced radioactivity,"
and for thorium simultaneously by Rutherford and
termed " excited radioactivity." With the explanation
of the property the original names have largely fallen
into disuse. We shall now confine ourselves to the case
136
THE ACTIVE DEPOSIT 137
of radium. Any object left in the immediate neigh-
bourhood of a radium salt becomes radioactive, but after
it is removed the radioactivity decays away rapidly
and almost completely, abnormally at first, but sub-
sequently more regularly, with a half-value period
approaching thirty minutes. The temporary activity
so " induced " consists of a-, ^-, and 7-rays. The
activity exists as an invisible film or deposit over the
surface of the object rendered radioactive, for, by sand-
papering, the activity can be rubbed off and then is
found on the sand-paper. It is now customary in con-
sequence to refer to it as the " active deposit of radium."
This power is, strictly speaking, not a property of
radium itself, for if the radium is contained in a com-
pletely closed vessel — it does not matter how thin- walled
so long as it is air-tight — no radioactivity whatever is
produced outside. The first step in understanding the
nature of the phenomenon consisted in tracing it to the
action of the emanation of radium. In the ordinary
condition the emanation is always diffusing away to some
extent from radium salts unless they are contained in
air-tight vessels. The " active deposit " is the product
of the disintegration of the emanation. Just as radium
cannot exist without continuously producing the emana-
tion, so in turn the emanation cannot exist without
continuously producing this active deposit. In any
vessel containing radium emanation this body is being
continuously deposited on the walls of the vessel, so
that if the emanation is at any time blown out, the
active deposit remains behind. Radium expels one
a-particle and changes into the emanation. The emana-
tion expels a second a-particle and changes back again
into a solid, or at least into a non-gaseous form of
matter, the first of the " active deposit " group. The
latter in turn expels more a- and also /S-particles, and
so the course of successive disintegrations goes on.
In the active deposit itself at least three changes follow
one another with great rapidity, so that the analysis
of them proved a complicated task.
138 THE SUCCESSIVE CHANGES OF RADIUM
The Active Deposit of Radium.
You know that if a moisture-laden atmosphere is
sufficiently chilled, the vapour of water condenses
directly into the solid form, and a snowstorm results.
Something of this kind is always happening in an atmo-
sphere containing the radium emanation. Every second
two out of every million of the atoms of emanation dis-
integrate, expelling «-particles and leaving a solid
residue, so that there is a sort of continuous snowstorm
silently going on covering every available surface with
this invisible, unweighable, but intensely radioactive
deposit. Unlike snow, however, the particles of this
active deposit are charged with positive electricity,
so that if two surfaces are provided, one charged nega-
tively and the other positively, the deposit is attracted
almost entirely to the negatively charged surface. The
other surface repels the particles and so does not get
coated. By making the negatively charged surface
very small the active deposit can be almost entirely con-
centrated upon it. This enables me to show you more
effectively the production of the active deposit from the
emanation and some of its chief properties. The
separation of the non-volatile product of a volatile
parent or emanation by this use of a negatively charged
surface is a very simple operation, much more so than
when the parent substance is non- volatile and the recoil
of the product is used to effect its separation and con-
centration on a negatively charged surface, as discussed
on p. 104.
It would take us too long and too far if we attempted
first to study these properties, and then tried from them
to deduce their explanation. It must suffice if I give
you first the explanation of the facts according to the
theory of atomic disintegration and then illustrate as
many of the points in it as possible experimentally.
I have said that after the disintegration of the emana-
tion at least three successive disintegrations, following
one another rapidly, occur. The bodies produced are
RADIUM A, B AND C 139
referred to as radium A, radium B, radium C, in
order to avoid the necessity of inventing a host of new
names for bodies having such fleeting existence (Fig. 30),
Radium. Emanation. Radium A. Radium B. Radium C.
"■ » ''
Active deposit of rapid change.
2,500 years. 5"6 days. 4-3 minutes. 38 -5 minutes. 28 •! minutes.
Fig. 30.
As before, the presumed atomic weights are placed
inside the circles corresponding with the successive
products. The periods of average life are placed below.
The symbol (/3) here - and throughout indicates that
/S-rays are expelled, but that they are not the normal
penetrating ^S-rays, but rays akin to the cathode-rays
in their low penetrating power and low velocity. They
only come into evidence in special experiments, and are
not of great general importance. The first body pro-
duced from the emanation, radium A, changes with great
rapidity with a period of average life of 4-3 minutes,
expelling an «-particle. The body radium B resulting
undergoes a change which was at first thought to be
entirely " rayless." Neither a- nor /9-rays of the ordin-
ary kind can be detected, although a very feebly pene-
trating jS-ray is produced, which we need not further
consider. The period of this substance is 38-5 minutes.
The body produced, radium C, changes, expelling both
a- and /3-particles and 7-rays also. The period is 28-1
minutes. It is probable that this change is complex
and that the /3- and 7-rays are given off in a separate
change to that in which the a-rays result. This point
will be dealt with later.
The Radiations from the Active Deposit.
We started our description of the rays of radium
with the statement that they consisted of a-, yS-, and
140 THE SUCCESSIVE CHANGES OF RADIUM
7-rays. One of the most interesting points of the above
scheme is to show that the /9- and 7-rays do not come
from radium itself, any more than they do from uranium
itself, but from the later products. It is loose, but con-
venient, to talk of the /3- and 7-rays of radium. Really
we mean the /3- and 7-rays of radium C. The emanation,
like radium itself, gives only a-rays. The whole of the
/3-rays result in the later changes of the active deposit.
We have seen that, freshly prepared from solution,
radium salts give only a-rays. The ^- and 7-rays
make their appearance only after the subsequent pro-
ducts have accumulated.
Experiments with the Active Deposit.
On the table there is a small glass vessel silvered
internally (Figs. 31 and 32) containing the emanation
from half a grain of radium bromide. It is arranged
so that steel knitting-needles can be inserted into the
emanation and withdrawn through a glass tube held
in a cork. The needle is connected to the negative pole
of the electric supply and the silver coating to the posi-
tive pole. If only the point of the needle is made to
project beyond the glass tube, the whole of the active
deposit can be concentrated on the point. Some hours
before this lecture a needle — we will call it No. 1 — was
so inserted, and by now its point should be coated to
its maximum degree of radioactivity with the products
of the disintegration of the emanation. After some
hours the products all arrive at the state of radioactive
equilibrium, in which the quantity is at its maximum for
all the products, radium A, radium B, and radium C,
as much of each changing as is produced from the emana-
tion. The disintegrations all going on together, the
wire should give a-, /S-, and 7-rays, the /3- and 7-rays
being as intense as those given from the half-grain of
radium bromide from which the emanation was derived.
Now I withdraw No. 1 needle from the emanation, and
with the room darkened we will examine its active
deposit.
Fig. 32. — Apparatus for obtaining the Active
Deposit of Radium.
To face p. 141
EXPERIMENTS WITH ACTIVE DEPOSIT 141
To detect the a-rays we will use a glass translucent
screen, thinly coated with phosphorescent zinc sulphide
on one side. I bring the point of the needle gradually
near the coated side of the screen. As soon as it comes
within a distance of three inches the screen lights up,
and when the point is only a little distance removed
from the screen a most brilliant phosphorescence is
produced. Now if I interpose between the wire and
the screen a single sheet of paper, the effect practically
Fig. 31.
entirely ceases. The a-radiations producing this effect
come both from radium A and from radium C.
To detect the y9-rays we will use an ordinary card-
board X-ray screen of barium platinocyanide. Bring-
ing the needle behind the screen, so that the rays
have to penetrate the cardboard, you observe the screen
lights up as brightly as with half a grain of radium
bromide itself. In the dark I happened actually to
touch the back of the screen with the active needle-
point, and in so doing some of the active deposit has
11
142 THE SUCCESSIVE CHANGES OF RADIUM
been transferred to the back of the screen. You can
see where the back of the screen was touched, because
this spot still glows though the needle has been removed.
If now the needle is again presented to the back of
the X-ray screen with thin pieces of metal foil inter-
posed, you see that the rays are only slightly stopped
by having to traverse the foil. When a piece of thick
lead sheet is interposed, a faint luminosity on the screen
still remains produced by the 7-rays. In fact the active
needle-point gives all the penetrating rays given by half
a grain of radium bromide.
It is now several minutes since the needle was removed
from the emanation. If we now again examine the
a-rays you will notice they already are very perceptibly
less intense than at first. Practically all the radium A,
of which the period of average life is only 4-3 minutes,
has already disintegrated, and in consequence the a-rays
now come only from the radium C, and are only half as
intense as at firsts
Radium A.
Now if, instead of exposing the needle to the emana-
tion for some hours so as to allow all the successive
products time to be produced, we expose it to the emana-
tion for a very short time, say for five minutes by the
watch, we shall get quite a different set of effects. Here
is a new needle; we will call it No. 2. Before putting
it in I will test it with the screen to show you that at
present it is an ordinary needle, not at all radioactive.
We will let it stay in the emanation, connected to the
negative pole as before, for five minutes and withdraw
it, and test its a-rays immediately, exactly as before.
You observe that it is already giving a-rays abundantly.
Comparing it with No. 1, the two are now very similar
in their a-ray-giving power, No. 1 being only slightly
the better. The a-rays from No. 2 come almost entirely
from radium A, for there has not yet been time for any
appreciable quantity of radium C to be formed. The
a-rays from No. 1 come entirely from radium C, and this
EXPERIMENTS WITH ACTIVE DEPOSIT 143
radiation has not yet had time appreciably to decay.
Let us, however, test their yS-rays. You observe that
No. 2 gives no yS-rays worth considering, whereas No. 1
still gives y8-rays in practically undiminished intensity.
Radium A gives no ^S-rays, and as there is no appreciable
quantity of radium C formed there yet, the consequence
is that No. 2 wire gives no /3-rays.
I can show you at this stage a very striking experi-
ment with another needle, No. 3, which has been in the
emanation a few minutes. I take it out and draw the
point once through a piece of emery-cloth and expose
the latter to the zinc sulphide screen. You observe that
a single rub has removed a large part of the active de-
posit from the needle and transferred it to the emery-
cloth, so that the latter makes the screen glow almost
as brilliantly as the needles themselves.
Radium B and C.
Now we will contrast the decay of the activity of the
needles Nos. 1 and 2. The activity due to radium A
by itself decays very rapidly, half disappearing every
three minutes. The consequence is, if we now again
test the a-rays of No. 2, we shall find they have already
nearly disappeared, whereas No. 1 still continues to
give a-rays at about the same strength as it did when
last examined. In ten minutes the «-rays of No. 2
practically disappear.
It is thus not difficult to give you a certain amount
of experimental evidence in favour of the conclusion
that the first change of the active deposit is a very lapid
one in which a-, but no yS-rays are expelled, and that
this is followed by a less rapid change in which both
a- and yS-rays are expelled. It is more difficult to give
you in a lecture satisfactory evidence of the existence
of radium B, a body not itself giving rays, intermediate
between the first and second changes in which rays are
expelled. If we examine carefully the decay of th e a-
and ^-rays of wire No. 1, in which at first all these pro-
144 THE SUCCESSIVE CHANGES OF RADIUM
ducts co-existed in equilibrium, we shall find, as already
shown, that for the first half-hour after removal from
the emanation the /S-rays suffer very little change and
then the regular decay begins. In the next half-hour
the /9-rays decay approximately to one-half their original
intensity, and the decay then goes on at this rate regu-
larly and continuously to the end. After two hours
they are only a few per cent, of what they originally
were, and in three or four hours they can no longer be
detected. The initial pause before decay begins is due
to the quantity of radium C being maintained, in spite
of the fact that it is disintegrating all the time, expelling
a- and )S-rays, by the disintegration of radium B. The
latter continues to supply new radium C to replace that
disappearing for the first half-hour or so after the
needle is removed from the emanation. Exactly the
same pause occurs in the decay of the «-rays. As we
saw with No. 1, within a very few minutes after the
needle was removed from the emanation the a-rays
had decayed to about one-half, owing to the disappear-
ance of the a-ray-giving radium A. Then, however,
little further change occurred. It is now about half
an hour since No. 1 was first tested, and the a-activity is
similar to what it was when last tested twenty minutes
ago. The a-rays of No. 2 have now almost completely
disappeared. If we continued to examine No. 1, we
should find, from now on, a rapid decay of both a- and
/3-rays at the same rate, so that at the end of the lecture
both will be much enfeebled, and by midnight both
will have ceased so far as we could tell by these rough
methods.
The Radiation from the Emanation.
Now that we have finished with the emanation
used in the preceding experiments, it is an interesting
experiment to show that itself it gives no ^-rays. If
we blow the emanation out into a U-tube of thin glass
cooled in liquid air, it is condensed in the cold tube.
THE RAYS OF THE EMANATION 145
The tube can then be sealed up to prevent the emanation
from escaping. The tube contains some phosphorescent
zinc sulphide and glows brightly owing to the a-rays
from the emanation inside. But if we hold the tube
against the X-ray screen, you can see that no penetrating
rays come from the tube. The emanation itself gives
no /3-rays, only a-rays. By the end of the lecture, how-
ever, sufficient radium C will probably have been
formed inside the tube to give an appreciable /S-radia-
tion. Owing to the existence of the intermediate body
radium B, there occurs a similar pause in the growth of
/3-rays from the emanation to that which, as we have
seen, occurs in their decay, after the emanation is taken
away. But in two or three hours the /S-rays from all
the needles will have decayed, and that from the sealed
U-tube will have reached a maximum.
The Later Slow Changes of Radium —
Radium D, E, and F.
This finishes this subject and brings us to the next.
What happens to radium C when it disintegrates ? Is
this the real or only the apparent end of the process ?
It is, in fact, a very long way from the end. Madame
Curie discovered that the rapid and almost complete
decay of the active deposit, at the end of a few hours
after removal from the emanation, is not in fact quite
complete. A very small residual radioactivity remains
and persists for years. The series of changes have now
entered on a stage which is as slow as the previous ones
were rapid. The next change requires almost as many
years as the last required minutes for completion.
The effect of these further changes is in consequence
extremely small, but they last a very long time. Con-
tinuing our diagram where it last ended at radium C,
the next stage is represented in Fig. 33.
The body produced from radium C, radium D, has
a period of many years. It is too early yet to state it
exactly. One recent estimate makes it twenty-four
146 THE SUCCESSIVE CHANGES OF RADIUM
years. No very important rays are given in its change.
/?-rays, however, result from the body produced from it,
which changes rapidly again with a period of only a
0-
Radium C. Endium D. Radium E. Radium F. Radium Q.
(Polonium.) (Lead?)
« . '
Active deposit of slow change,
28*1 minutes. 24 years (?) 7-5 days. 202 days.
Fig. 33.
few days. We shall pass over these intermediate changes
and consider the last known change of the series, that
of radium F, which has a period of average life of
202 days, in which an a-particle is expelled. Radium F
is the 'polonium of Madame Curie, having been separated
by her from pitchblende first before she discovered
radium.
Polonium.
A digression may here conveniently be made on what
is known about polonium, before its connection with
radium is considered. Chemically it resembles bismuth,
and was separated first from pitchblende in association
with the bismuth contained in the mineral. Its radio-
activity, which consists entirely of a-rays, slowly and
completely decays, so that a few years after it has been
prepared, the most intensely active preparations of it
lose practically all their activity. The work was carried
on by Marckwald in Germany, who discovered new and
simple methods of extracting polonium from the mineral
and worked up many tons of pitchblende for this sub-
stance. His careful chemical investigations of the
nature of the body made it clear that it was quite as
nearly allied in chemical nature to the element tellurium
as to bismuth, and he first proposed the name " radio-
tellurium " for it, which, however, with the elucidation
POLONIUM 147
of its identity with polonium, has fallen into disuse.
He proved that there is far less polonium in the mineral
even than radium. In a ton of mineral there is less than
a thousandth part of a grain of polonium, but the radio-
activity is correspondingly intense, and greatly exceeds,
so far as the a-radiation is concerned, that of pure radium
itself. The period of average life, 202 days, is deduced
by direct observation from the rate of decay of the radio-
activity.
Returning now to the consideration of radium C,
we saw that after its activity had decayed there existed
still a residual activity which is very feeble. This
steadily increases with time, and consists both of a- and
/3-rays, which, however, increase at different rates. The
a-rays are due to polonium, or radium F. These go
on increasing for the first two years and then a maximum
is reached, the amount of the radium F formed being in
equilibrium. The /3-rays, however, reach a maximum
much more quickly. The /3-ray product (radium E)
having a much shorter period, equilibrium is reached
in a few weeks. If at any time the active matter is
subjected to the chemical processes worked out by
Marckwald for the separation of polonium, the a-ray
body radium F can be separated from the other products,
and its activity then decays away completely at exactly
the same rate as in the case of polonium. Moreover,
it shows the property of being volatile at a temperature
of a bright red heat, which is the basis of one of the
methods originally used by Madame Curie in separating
polonium from the bismuth in pitchblende. This is
merely a sketch of the evidence in favour of regarding
polonium as the last radioactive substance produced
in the disintegration of uranium.
The Ultimate Product of Radium.
One more step remains to be discussed, and then
this long story of continuous transformation is at an end.
What is the ultimate product ? When radium F or
148 THE SUCCESSIVE CHANGES OF RADIUM
polonium expels its a-particle, what is produced ?
The estimated atomic weight of polonium is 210, which
is deduced by subtracting from the atomic weight of
radium (226) the weight of the four atoms of helium
known to be expelled in the form of a-particles. This
agrees well with its chemical nature, for there is a
vacant place in the periodic table for an element, the
next heavier than bismuth (atomic weight, 208-5), and
this element would be chemically analogous to tellurium.
The expulsion of an a-particle would further reduce the
atomic weight four units, leaving a residue of atomic
weight 206. What is it ?
Now, if this is really the final product and not merely
a very sloAvly changing substance, the formation of
which in proportion to the degree of slowness of the
change would be difficult experimentally to detect, then
it follows that the ultimate product must accumulate in
quantity indefinitely with time in the minerals contain-
ing the elements of the uranium-radium series, and must
therefore be a well-known common element. Lead has
the atomic weight of 207-2, and bismuth, 208-0. The
next known element is thallium (204), and then comes
mercury (200).
Lead is found in all the common minerals containing
uranium in considerable quantity, and there is also
evidence that the older the geological formation from
which the mineral is obtained, the greater the percentage
of lead present. Recently a uranium mineral, autunite,
has been found containing no chemically detectable
quantity of lead. But then the same mineral contains
only an excessively minute trace of helium, and less
than its full equilibrium amount of radium. There is
every reason to believe that its formation as a mineral
has occurred in quite recent times.
This question has now been settled by indirect means,
and there is no longer room for doubt that lead is the
ultimate product of uranium. This evidence, however,
may be deferred. The method of settling it directly is
to study the change of polonium, separated from enor-
THE TWO URANIUMS 149
mous quantities of pitchblende, by the aid of the spec-
troscope, and on this task Mme. Curie and her colleagues
have for long been engaged, but as yet without definite
proof that lead is the product.
Uranium I and Utjanium II.
A variety of evidence, some of which may be dealt
with more profitably later, has lately established the
conclusion that the change suffered by the uranium
atoms when the a-particles are expelled, is not, as first
supposed, a single change. The substance uranium,
which chemists have hitherto considered an element,
differs from every other known substance expelling
«-rays, in that, per atom disintegrating, two a-particles
are expelled instead of one. Moreover, these two
a-particles are 'expelled at slightly different initial
velocities, with the result that the " ranges " of the
two sets of a-rays in air are slightly different (see p. 164).
Most probably the two a-particles are not expelled from
the uranium atom simultaneously but successively. In
consequence, what chemists hitherto have accepted as
a single element is, in reality, a mixture of two, chemi-
cally so much alike that they have not yet been separ-
ated, the first having the atomic weight 238-5, and which
has been termed provisionally uranium I; the second,
resulting after the expulsion of the first a-particle, having
the atomic weight 234*5. It has been termed uranium
II. It is probable that this uranium II is present in
relatively very insignificant proportion by weight,
although it contributes one-half of the total a-radiation.
Its period of life can only be estimated from very indirect
and incomplete data at the present time. The more
slowly a radioactive substance changes the shorter the
range of the a-particles it expels, and so from the range
of the rays an estimate of the period of average life
may be formed. This estimate, such as it is, attri-
butes a period to uranium II of about two million
years.
150 THE SUCCESSIVE CHANGES OF RADIUM
Numerous similar examples of elements identical
chemically, but differing in radioactivity, are now
known. These are called isotopes or isotopic elements.
Uranium X and Uranium X, (Brevium).
Even more recently it was first predicted and then
shown that uranium X is not a single substance. Ura-
nium X gives two kinds of /S-rays, one of low velocity
and comparatively non-penetrating — i.e., (13-) rays
(compare p. 139) and ordinary high velocity /3-rays.
® ^'^^^ •PC&y) @ @ ©
(238) >(234) ^^34) ^(234J ^(230) ^(226) >
Uranium I Uranium X^ Uranium X 2 Uranium II Ionium Radium
8,000,000,000 35.5 days 1.65 minutes 3,000,000 100,000 2,500 years
years years (?) years
^22) ^(218 j >- r214j >- (214.") ^ M214J >
Emanation ,RadiumA Radium B Radium C Radium C^
'" ^ Active Deposit of Rapid Change
4-3mins. 38-smins. 28-1 mins. One millionth
,^ , , . ,^ of a second (?)
^lOj ^(210j >- (210) ^ (2O6)
Radium D Radium E Radium F Radium G
^ (Polonium) (Lead)
Active Deposit of Slow Change
24 years 7-25 days 202 days
Fig. 34.
These have been shown to originate from two distinct
substances successively produced, and which are called
uranium Xj and uranium Xg. Uranium I expels an
«-particle and changes into uranium Xj, which has a
period of average life of 35-5 days, and expels, not the
penetrating /S-rays of " uranium X," but the feeble and
unimportant (/8)-radiation. Its product, uranium Xg,
sometimes called brevium, has the very short period of
average life of only 100 seconds, and, in its change, the
powerful penetrating /3-rays are expelled. Uranium
Xj, after its change, is believed to become uranium II.
COMPLETE URANIUM SERIES 151
The latter in its change expels an a-particle and is
believed to produce ionium. Uranium Xj and uranium
Xo, unlike uranium I and uranium II, can be separated
from one another by chemical methods. These new
discoveries, although of highest theoretical importance,
make very little practical difference to the results, which
for almost all ordinary purposes are precisely what they
would be were the simple scheme, shown in Fig. 29,
actually the one followed.
Radium C and Radium C.
Lastly, there is indirect evidence that radium C
consists of two successive products, distinguished as
radium C and radium -C, the first giving the ^- and
7-rays in its disintegration, and producing the second,
which has a period of average life of only a millionth of
a second, and changes, emitting an a-particle, into
radium D (see p. 202).
Fig. 34 shows so far as it is at present known the
complete disintegration series of uranium.
CHAPTER X
RADIOACTIVITY AND THE NATURE OF
MATTER
Ratio of Quantities of Polonium and Radium
IN Minerals.
From the law, which has already been found so useful,
we can calculate the ratio of the quantities of radium
and polonium that exist together in a mineral from
their periods of average life. The period of average life
of radium is 4,500 times that of polonium, so that there
must be 4,500 times more radium than polonium in
minerals. A good pitchblende with 50 or 60 per cent,
of uranium in it contains about an ounce of radium
in 150 tons. The same quantity of polonium would
therefore be contained in about 700,000 tons. The
whole output of the Joachimsthal mine per annum,
reckoned as 15 tons, contains about one hundredth of
a grain of polonium. This is borne out by Marckwald's
experiments, already referred to.
Let us apply the law not only to radium and polo-
nium, but to the whole list of known transition-forms
existing as products of uranium. In the table this has
been done. The first column gives the name of the
substance, the second its period of average life, and the
third its relative quantity in minerals, the quantity
of uranium being considered 1,000,000,000. If these
numbers are taken to refer throughout to milli-
grams (1 milligram is about yV of a grain), then since
1,000,000,000 milligrams is roughly a ton, the quan-
tities refer to an amount of mineral containing one ton
of the element uranium.
152
COMPOSITION OF A URANIUM MINERAL 153
TABLE.
Period.
Uranium I, 8,000,000,000 years
Uranium X .. 35-5 days.
Uranium X2
Uranium II, 3
Ionium
Radium
Emanation
Radium A
Radium B
Radium C
Radium D
Radium E
Radium F
tPolonium)
1-6 minutes,
,000,000 years (?).
100,000 years.
2,500 years.
5-6 days.
4-3 minutes.
38'5 minutes.
28'1 minutes.
24 years (?).
7-5 days.
202 days.
Quantity.
1,000,000,000 mg. (=1 ton).
One eightieth mg.
l/250,000th mg.
400 grams (?).
12-5 grams.
312-5 mg.
One five-hundredth mg.
One millionth mg.
Nine millionths mg.
Seven millionths mg.
3 mg. (?).
One four-thousandth mg.
One fourteenth mg.
These respective quantities in the last column emit
a similar number of «-particles per second in the eight
cases where a-particles are expelled at all, and so pro-
duce similar radioactive effects. This is an illustration
of the compensating principle I spoke of earlier, that
the quantity of a radioactive substance divided by its
life, not the quantity only, gives a measure of its radio-
active effects. It can readily be calculated that the
actual amount of radium A used in our experiments,
which produced powerful and striking effects on the
phosphorescent screen, was much below one ten-
milHonth of a milligram, or below one thousand-
millionth of a grain. For it was derived from 30 mg. —
i.e., half a grain of radium bromide. Yet while it lasts
it comes into evidence through the energy of the a-
particles expelled in its rapid disintegration no less
than any of the other products.
Impossibility of Concentrating Many of the
Products of Disintegration.
The table brings out clearly that radium is but one of
many radioactive substances in uranium minerals,
which would be of value if they could be extracted.
Uranium II, ionium and radium D, all possess sufii-
154 RADIOACTIVITY AND NATURE OF MATTER
ciently extended periods of life to repay recovery.
Ionium gives only very feebly penetrating a-rays, and
so would not be so generally useful as radium, whereas
uranium II and radium D both, being followed by short-
lived products which give ^-rays, would be of great
general utility. The reason which has precluded the
practical separation of these substances in the past is a
general one, which has proved to be of the highest
philosophical significance in the chemistry of these new
ephemeral elements. They all so closely resemble one
or other of the known elements that the separation is
impossible. The resemblance between radium and
barium is of great practical utility, because these two
elements, though very closely alike in chemical nature,
can be separated from each other after they have first
been separated from every other element. Taking them
in order, uranium II cannot yet be separated from
uranium I, ionium cannot be separated from thorium,
nor radium D from lead. Lead, as has been stated, is
almost always present in considerable quantity in ura-
nium minerals, and so usually is thorium, but to a much
more variable extent. Hence, though it is easy to
separate radium D from the mineral with the lead, it is
at present useless practically, as it cannot be concen-
trated from the lead. By choosing suitable minerals
like secondary pitchblendes, which do not contain
ponderable quantities of thorium, intensely active pre-
parations of ionium can however be separated. It is
at present the only one in the uranium series likely to
become useful, and its lack of penetrating rays is a
serious drawback. Polonium, with its period of less
than a year and its absence of penetrating rays, hardly
repays extraction, except for purely scientific investiga-
tions. There is, however, another disintegration series,
that of thorium, which offers a better chance of
providing an efficient substitute for radium, and this
series will therefore be briefly considered in a later
chapter.
THE RARITY OF ELEMENTS 155
Increase of Radioactivity of Radium with
Time.
The increase of the radioactivity of radium after it is
prepared is due to the steady growth of the products
undergoing further disintegration. As we know, when
freshly prepared from solution, the activity of radium
is due solely to its own disintegration and consists of
a-rays. After four weeks the first four products accu-
mulate to their equilibrium, and the activity now
consists of a-, yS-, and 7-rays, the a-rays being four
times as great as initially. It is not difficult to see that
the later slow changes must also cause a very slow
further continuous increase of all these types of rays,
due to the growth of radium E and polonium from
radium D. These considerations are embodied in the
following table giving an analysis of the total radio-
activity of a radium preparation, kept in a sealed vessel
so that none of the products escape, at different periods
since preparation:
a-PARTICLES.
|8-P ARTICLES.
I. Freshly prepared.
1 (due to radium itself)
0
II. After one month.
4 (1 due to radium)
1 or 2
(1 due to emanation)
(due to Ra C)
(1 due to radium A)
(1 due to radimn C)
II. After a century.
5 (as in II and 1 due
2 or 3
to radium F)
(1 due to Ra Eg)
The Rarity of Elements.
The idea, which is a necessary consequence of the
atomic disintegration theory, that fixed definite relation-
ships must exist between the quantities of elements
formed from one another — for example, between ura-
nium, radium, and polonium — forms the first indication
that physical laws may exist regulating the relative
abundance and scarcity of elements in Nature. Gold
and platinum, for example, are valuable or rare metals,
and we do not know why. Radioactive bodies like
156 RADIOACTIVITY AND NATURE OF MATTER
radium are rare because of the rapidity with which they
are changing. The degree of radioactivity of an element
being proportional to the rate at which it is changing, it
follows that radioactive elements are scarce and valuable
in proportion to their radioactivity. In this case degree
of radioactivity is a physical measure of value or rarity.
It is, for example, so far as we can see, an impossibiUty
that an element like radium will ever be found in greater
abundance in any minerals than in those already known.
Naturally, in the consideration of some of these ques-
tions of general interest upon which we are now entering,
we are, be it said, in sharp contrast to almost everything
we have dealt with in the subject up to now, frankly
speculating. But it is helpful and legitimate to specu-
late upon how far, if at all, the process of atomic disin-
tegration, discovered for the radio-elements, applies to
the case of elements not radioactive, of which there is
as yet no positive evidence that they are changing at
all. The workers in radioactivity have within their
province explored thoroughly the process of atomic
disintegration. They have made clear the laws it
follows, they have measured the rates at which it occurs,
and they have established what may be termed its in-
evitableness or independence from all known influences.
But there is no reason why the process should be limited
in its scope to the somewhat special phenomena which
led to its discovery.
The Cukrency Metals.
It is, for example, natural to inquire whether the
scarcity of elements like gold is fixed by the operation
of similar physical laws to those which regulate the
rarity of radium. The race has giown used from the
earliest times to the idea that gold is a metal possessing
a certain fixed degree of value, enabling it to be used
safely for the purposes of currency and exchange. It is
no exaggeration to say that the whole social machinery of
the Western world would be dislocated if gold altered
CURRENCY METALS 157
violently in its degree of rarity — if, for example, in some
hitherto unpenetrated fastness of the globe a mountain
of gold came to be discovered.^ Is there not at least a
strong presumption that this is really as contrary to the
operation of natural law as the discovery of a mountain
of pure radium would be ?
It may, I think, be taken for granted that an element
changing more rapidly than uranium, for example —
that is, with a period of average life of less than
8,000,000,000 years — is not likely to be much more
plentiful in nature than uranium, and therefore that all
the common elements — lead, copper, iron, oxygen,
silicon, etc., etc. — have periods of average life of many
thousands of millions of years. So far, the traditional
view that the elements' are permanent and unchanging
is substantially correct. At the same time, we cannot
but recognise that inevitably the effects of atomic dis-
integration, too slow to be otherwise detectable, would
result in the accumulation of the more stable and
longest-lived elements at the expense of the others,
resulting in some sort of equilibrium in which the
relative abundance of the elements was proportional to
their respective periods of average life. For example,
the ratio between the relative abundance of gold and
silver is roughly but pretty certainly known, owing to
these metals being employed for currency purposes
from the earliest times. It is at least a possible view
to take that the elements gold and silver belong to the
same disintegration series, both changing very slowly,
but the gold many times more rapidly than the silver.
Obviously we are only at the beginning. But already
it cannot be gainsaid that the interest and importance
of this process of atomic disintegration is not confined
to radioactivity only or even to physical science. It
extends into almost every region of thought.
1 Since thesf' words were first written the whole social machinery of
the Western World has been dislocated by violent alterations in the
purchasing power of gold, and it has been shown to be no longer a safe
medium f^r currency. (Compare A Fraudulent Standard, A. T. Kitson.
London: P. S. King and S.n, Ltd., 1917.)
12
158 RADIOACTIVITY AND NATURE OF MATTER
The Nature of Atoms.
I now propose considering briefly another question
of general philosophical interest in connection with the
recent advances of physical science. Naturally the dis-
coveries in radioactivity have not been made without
influencing considerably our ideas on the ultimate nature
of atoms. In some points older conceptions have had
to be modified, while in others these conceptions have
been strangely confirmed. It has always been a matter
for remark, considering the myriads of individual atoms
which go to make up the smallest perceptible quantity
of matter, that there are so few different kinds. The
number of atoms which go to make up this world, for
example, would run into at least fifty-four figures, yet
among them all there are less than a hundred different
varieties. Moreover, it has come to be regarded as one
of the greatest philosophical generalisations of physical
science that all the atoms of one kind, that is to say of
one element, are, at least as far as was known up to the
beginning of the present century, completely similar in
character. There is, for example, not the shadow of
distinction between gold found in the Klondyke, in
Australia, or in S. Africa. Not only so, but we have
learned from the spectroscope that this similarity of
nature extends throughout the whole universe. In this
connection, both to set forth the idea and to illustrate
the deductions which have been drawn from it, I cannot
do better than to quote a celebrated utterance of Clerk
Maxwell to the British Association in 1873. I may
remark that Clerk Maxwell throughout used the word
molecule in the sense of " atom " as this word is em-
ployed by the chemist, and throughout these lectures.
" In the heavens we discover by their light, and by
their light alone, stars so far distant from each other
that no material thing can ever have passed from one
to another; and yet this light, which is to us the sole
evidence of the existence of these distant worlds, tells
A QUOTATION FROM CLERK MAXWELL 159
us also that each of them is built up of molecules of the
same kinds as those which we find on earth. A molecule
of hydrogen, for example, whether in Sirius or in Arc-
turus, executes its vibrations in precisely the same time.
" Each molecule therefore throughout the universe
bears impressed upon it the stamp of a metric system
as distinctly as does the metre of the Archives at Paris,
or the double royal cubit of the temple of Karnac.
" No theory of evolution can be formed to account
for the similarity of molecules, for evolution necessarily
implies continuous change, and the molecule is incapable
of growth or decay, of generation or destruction.
" None of the processes of Nature, since the time when
Nature began, have produced the slightest difference in
the properties of any molecule. We are therefore unable
to ascribe either the existence of the molecules or the
identity of their properties to any of the causes which
we call natural.
" On the other hand, the exact equality of each mole-
cule to all the others of the same kind gives it, as Sir
John Herschel has well said, the essential character of a
manufactured article, and precludes the idea of its being
eternal and self-existent.
" Thus we have been led, along a strictly scientific
path, very near to the point at which science must stop;
not that science is debarred from studying the internal
mechanism of a molecule which she cannot take to
pieces, any more than from investigating an organism
which she cannot put together. But in tracing back
the history of matter. Science is arrested when she
assures herself, on the one hand, that the molecule has
been made, and on the other, that it has not been made
by any of the processes we call natural.
" Science is incompetent to reason upon ^^he creation
of matter itself out of nothing. We have reached the
utmost limits of our thinking faculties when we have
admitted that because matter cannot be eternal and
self-existent it must have been created."
You will admit that, in the light of all that has trans-
160 RADIOACTIVITY AND NATURE OF MATTER
pired in the forty-five years since Maxwell used these
words, science has advanced far. The concluding words
of the address are even more striking from this point of
view.
" Natural causes, as we know, are at work, which tend
to modify, if they do not at length destroy, all the
arrangements and dimensions of the earth and the whole
solar system. But though in the course of ages catas-
trophes have occurred and may yet occur in the heavens,
though ancient systems may be dissolved and new
systems evolved out of their ruins, the molecules out of
which these systems are built — the foundation-stones
of the material universe — remain unbroken and un-
worn."
Before we dwell upon the modifications that have
been made in this point of view, let us rather consider
the chief basis of the argument, namely, that all the
atoms of any one element are exactly alike. On this
fundamental question the evidence to-day is far more
complete and definite than it was in 1873. Recent
developments in connection with isotopes have modified
our point of view, but for the moment we may neglect
this special advance.
We no longer regard the atom as a simple thing. On
the contrary, we now look upon it as an almost infinitely
complex piece of mechanism. The late Professor Row-
land, of Baltimore, once made the remark that a grand
piano must be a very simple piece of mechanism com-
pared with an atom of iron. For in the spectrum of
iron there is an almost innumerable wealth of separate
bright lines, each one of which corresponds to a sharp
definite period of vibration of the iron atom. Instead
of the hundred-odd sound vibrations which a grand piano
can emit, the single iron atom appears to emit many
thousands of definite light vibrations. Two pianos
would be regarded as in perfect tune together when there
was a comparatively rough approximation of period
between the various notes. Whereas by the spectro-
scope a difference in " tune " or period in the vibra-
THE PERFECTION OF THE ATOMS 161
tions emitted by different atoms of only one part
in many millions would be easily detectable, and no
such variation exists. In a similar vein Professor
Schuster, referring to the broad teachings of the spectro-
scope, has compared the atoms of the same element to
an innumerable number of clocks all wound and regu-
lated to go at the same period. If all these clocks were
set at the same time, not one of them would vary by a
single second even after many many days. No clock-
maker could make such clocks. Yet these almost
infinitely complicated pieces of mechanism we call
atoms are turned out by Nature with such undeviating
accuracy and fidelity that in all the myriads in existence
there are less than a hundred different kinds known.
The Velocity of a-PARTiCLES.
We can, however, from the point of view of recent
researches in radioactivity, push this idea even one step
further, to the case of atoms actually in the condition
of breaking up. We have seen that it is a property of
the a-rays to possess a very sharp and definite range.
In a beam of homogeneous «-rays passing through a
homogeneous absorbing medium the number of a-par-
ticles suffers little or no diminution until the extreme end
of the path is reached, and then they cease altogether.
Just without the extreme range, there is absolutely no
effect perceptible, while just within this range, the effect,
per small element of path, is at the maximum. Every
«-particle expelled from the radio-element in the same
change travels exactly the same distance before it ceases
to be detectable, and, as Rutherford has shown by direct
measurement of the magnetic and electric deviation, is
expelled at the same velocity.
In the table following, the approximate initial velor
cities of the a-particles from the changes in the uranium
series have been collected, together with their " ranges "
or distances in millimetres they will penetrate in air at
15° C. and 760 mm. of mercury pressure.
162 RADIOACTIVITY AND NATURE OF MATTER
a-PARTICLE
Period.
Velocity
FROM
(miles per second
i Range
Uranium I,
. 8,000,000,000 years
8,800
.. 25
Uranium II,
3,000,000 years
9,300
. 29
Ionium
100,000 years
9,400
. 30
Radium
2,500 years
9,600
.. 33
Emanation
5-6 days
10,400
. 42
Radiium A
4-3 minutes
10,900
. 47-5
Radium C'
. 1/1, 000 ,000th sec.
(?)
12,400
. 69-5
Radium F
202 days
10,200
. 37-7
The atom thus retains its role of a perfect piece of
mechanism even up to and during the moment of its
dissolution. So exactly alike are all the atoms of the
same radioactive element, that when the break-up occurs
the velocity with which the fragments of the atom, or
a-particles, are expelled is exactly the same in each case.
We may liken the disintegration of an element to the
bursting of shells, in which the fragments of the different
shells all are expelled with the same velocity. Certainly
no shells ever constructed would answer this require-
ment. Truly, in the words of Sir John Herschel,
the atom bears the essential character of a manufac-
tured article, but of a degree of perfection humanly un-
attainable.
But with regard to the process of manufacture and of
the cause of this undeviating fidelity to a few types,
what a revolution of thought has taken place in the last
few years ! The evolution, or rather devolution, of
matter, its continuous change, the generation and
destruction of atoms — all of the things which seemed
impossible in Clerk Maxwell's day — we know to be
going on before our eyes. It is true the processes call
for periods of time so vast, even in the most favour-
able cases, that the physicist of a generation ago would
have dismissed them as physically inconceivable. Yet
these periods are to-day actually determined by direct
measurement in the laboratory.
THE SURVIVAL OF ELEMENTS 163
Stability and Survival of Elements.
Instead of regarding the hundred or less elements
which exist to-day as manufactured, created, once for
all time, we rather regard them as existing because they
have survived. All other forms less stable than those
we recognise as elements have been weeded out. Over
sufficiently great periods of time the rarity or abundance
of an element must be controlled by its degree of in-
stability or stability. Probably for every stable atom
many unstable ones could be, even are being, formed.
But only the stable forms can accumulate in quantity
and become known to us as ordinary chemical elements.
We have seen that the rarest of such in all probability
must have a period of thousands of millions of years,
while for the more common elements, if they are chang-
ing at all, periods of billions of years may be anticipated.
At first glance only, the material universe gives the
impression of a permanent and finished creation. In
reality the now familiar remorseless operation of slow,
continuous change moulds even " the foundation-stones "
themselves. By this last step the doctrine of evolution
has become universal, embracing alike the animate and
inanimate worlds. But whereas in the former slight
changes of environment effect the profoundest modifica-
tions, in the latter the controlling factors still remain
absolutely unknown. By the spectroscope a partial
material survey of the whole universe has been ren-
dered possible, and what we find is everywhere an essen-
tial similarity of composition. For example, there is no
evidence that in the sun or stars large quantities of
elements unknown to us exist. The reason why some
atoms are stable and others are not is a mystery we
have not yet begun to probe. Yet this question, to us
only of academic interest and possibly somewhat remote
at that, will, as we shall soon come to see, be one of
life and death to the inheritors of our civilisation.
164 RADIOACTIVITY AND NATURE OF MATTER
Connection between Range of a-RAYS and
Period.
A very interesting development may now be men-
tioned, which has resulted in a connection being estab-
lished between the ranges or velocities of the various
types of a-rays, and the periods of life of the atoms
from which they are derived. As a general rule — not,
it is true, entirely without exceptions, but possibly the
exceptions may prove to be only apparent — the more
rapidly a radioactive substance disintegrates, or the
shorter its period of average life, the greater is the
velocity with which the «-particle is expelled from
the atom, and the greater therefore is the range of the
a-particle. Thus, the most stable radio-elements, ura-
nium and thorium, give a-rays having the lowest ranges,
and the low range of the a-rays of ionium was for long
the only evidence that its period must be very long.
The greatest ranges occur in the short-lived " active
deposit " products. The very long ranges of the a-rays
of radium C (69-5 mm.), and of the corresponding thorium
C (86 mm.), is generally explained by the supposition that
the real atoms giving these rays have periods of the order
of only a millionth of a second, and therefore that it is
impossible to separate them from their parents, which
thus appear to be giving rays which in reality come
from their products. This will be referred to again.
Latterly, this generalisation has been put into stricter
form by the discovery that if the logarithm of the period
is plotted against the logarithm of the range or of the
velocity, straight lines result for each of the three known
disintegration series. The three straight lines are
parallel to but not identical with one another. The
reason for this is still obscure. Some mathematical
connection exists between the two quantities, and that
is all that can yet be said. On the other hand, it has
been found possible to calculate approximately some of
the unknown periods — like that of ionium, so estimated
PLEOCHROIC HALOS 165
at 200,000 years, for example, from the ranges of the
a-rays by means of this relation before it was directly
determined to be 100,000 years.
For long it was known that uranium was exceptional
in that it appeared to give out two a-particles per atom
disintegrating instead of one, as in all other cases. A
very careful investigation revealed the fact that the
ranges of these two sets of a-particles were riot exactly
alike. One set, those from uranium I, presumably, have
a range of 25 mm., and the other set, those from its
shorter-lived product, uranium II, presumably, a range
of 29 mm. The period corresponding with 29 mm. of
range is, in the uranium series, two million years, and
this is the main evidence for believing that such a
product, uranium II as' it is called, exists, and that it
has so far not been separated from uranium because
of the identity of the chemical properties of the two
elements.
Pleochroic Halos.
The account given in this chapter and in Chapter III.
of the many extraordinary properties of the a-particle
would be incomplete if another natural phenomenon in a
totally distinct field were omitted. The a-, in common
with the other rays from radioactive substances, have
the power of darkening glass and other transparent
materials such as mica after long exposure. Indeed,
the colours of many natural gems have been traced to
the effect of such rays from naturally occurring radio-
active materials in the earth, operating over immense
periods. Sir William Crookes artificially coloured a
large colourless diamond an intense green by exposing
it for some weeks to the rays from a pure radium
compound.
Many other gems, usually found in a colourless state,
can similarly be made to assume the most varied colours,
the nature of which depend probably upon slight
chemical impurities present in the gem. Mica under
these circumstances becomes deeply stained and dark.
166 RADIOACTIVITY AND NATURE OF MATTER
Now, occurring in various natural micas, there are
sometimes found microscopic halos of darkening of
perfect circular outline, called pleochroic halos. These
have been very exhaustively studied by Professor
Joly, and the microphotographs shown in Figs. 35 and 36
are taken from a paper by him and Mr. Fletcher in the
Philosophical Magazine for 1910. Fig. 35 shows two of
these halos in a specimen of mica. Sometimes the halos
are made more visible by the use of polarised light, but
this is not always necessary. It can be shown, by suit-
ably sectioning the material, that the halos are true
spheres, and often at the centre a juinute microscopic
nucleus is visible. Professor Joly measured exactly with
the microscope the diameter of these halos, and found
them to correspond perfectly correctly with the " range "
of the «-particles from radium C, which in mica is
0-06 mm. He put forward the view that they were
due to a-particles, from radioactive material in the
central nucleus, darkening the mica over a sphere
bounded by the range of the a-rays. This conclusion
has been most brilliantly confirmed. It is possible to
find halos in various stages of development. Young and
incompletely developed halos often show only a central
" pupil " of only 0-013 mm. in radius. This corresponds
with the range of the shorter a-particles, due to uranium,
ionium, and radium itself. In later stages a distinct
" corona" appears of the full radius, 0-03 mm., which
is the range of the a-particle from radium C in mica.
And in particularly favourable cases it is possible to
see between them an inner ring of dimensions corre-
sponding with the intermediate range of the a-particles
of radium A. A much enlarged micro-photograph of
such a halo is shown in Fig. 36.
Ueanium and Thorium Halos.
Moreover, a careful search revealed other halos of
slightly greater radius than 0'03 mm. — viz., 0*038 mm. —
which corresponds with the range of the fastest a-par-
Fig. 35. — Thorium and Radium Halos in Biotite.
( X 150 Diameters.)
Img. 36.— Halo in Biotite. ( x 450 Diameters.)
Showing ring due to Radium A.
To face p. 166
URANIUM AND THORIUM HALOS 167
tide emitted in the thorium series. An examination of
them showed a course of development totally different
from that of the uranium halos. The successive states
in this case correspond with the a-rays of the ranges that
are emitted in the thorium series.
As a matter of fact the lower halo in Fig. 35 is due
to uranium and the upper one due to thorium. The
uranium halo is fully developed, so that the central
" pupil," though visible in the microscope, cannot be
seen in the reproduction. The thorium halo shows
faintly but quite clearly the corona due to the long
range rays of thorium C, the longest known. Still
other halos attributed to radium emanation without
the earlier members of the series have been observed.
It may be concluded that the nucleus at the centre
either contains uranium or thorium in minute quantity,
or has the power of occluding radium emanation from
water that has flowed through uranium minerals. But
the actual a,mounts of radioactive materials so put into
evidence are almost inconceivably minute and far
beyond the power of detection even by the most sensi-
tive electrical method. It has been estimated that the}'-
are due to the expulsion of sometimes less than 100
a-particles per year, continuing for several hundred
million years. The mica integrates these infinitesimal
effects throughout the ages so that at length they are
able to produce consequences visible to the eye. Until
this explanation was forthcoming, they had remained a
complete puzzle to the petrologist.
CHAPTER XI
RADIOACTIVITY AND THE EVOLUTION OF
THE WORLD
The Potentialities of Matter.
This interpretation of radium is drawing to a close, but
perhaps the more generally interesting part of it remains
to be dealt with. We have steadily followed out the idea
of atomic disintegration to its logical conclusions, so fai*
as they can at present be drawn, and we have found it
able to account for all the surprising discoveries that
have been made in radioactivity, and capable of pre-
dicting many, and perhaps even more unexpected, new
ones. Let us from the point of vantage we have gained
return to the starting-point of our inquiries and see what
a profound change has come over it since the riddle has
been read. Radium, a new element, giving out light
and heat like Aladdin's lamp, apparently defying the
law of the conservation of energy, and raising questions
in physical science which seemed unanswerable, is no
longer the radium we know. But although its mystery
has vanished, its significance and importance have vastly
gained. At first we were compelled to regard it as
unique, dowered with potentialities and exhibiting
peculiarities which raised it far above the ordinary run
of common matter. The matter was the mere vehicle
of ultra-material powers. If we now ask, why is radium
so unique among the elements, the answer is not because
it is dowered with any exceptional potentialities or
because it contains any abnormal store of internal energy
which other elements do not possess, but simply and
solely because it is changing comparatively rapidly,
168
POTENTIALITIES OF MATTER 169
whereas the elements before known are either changing
not at all or so slowly that the change has been unper-
ceived. At first sight this might seem an anti-climax.
Yet it is not so. The truer view is that this one element
has clothed with its own dignity the whole empire of
common matter. The aspect which matter has pre-
sented to us in the past is but a consummate disguise,
concealing latent energies and hidden activities beneath
an hitherto impenetrable mask. The ultra-material
potentialities of radium are the common possession of
all that world to which in our ignorance we used to refer
as mere inanimate matter. This is the weightiest lesson
the existence of radium has taught us, and it remains
to consider the easy but remorseless reasoning by which
the conclusion is arrived at.
Why Radium is Unique.
Two considerations will make the matter clear. In
the first place, the radioactivity of radium at any
moment is, strictly speaking, not a property of the mass
of the radium at all, although it is proportional to the
mass. The whole of the new set of properties is con-
tributed by a very small fraction of the whole, namely,
the part which is actually disintegrating at the moment
of observation. The whole of the rest of the radium
is as quiescent and inactive as any other non-radio-
active element. In its whole chemical nature it is an
ordinary element. The new properties are not con-
tributed at all by the main part of the matter, but
only by the minute fraction actually at the moment
disintegrating.
Let us next compare and contrast radiunt mth its
first product, the emanation, and with its original parent,
uranium. Uranium on the one hand, and the emanation
on the other, represent, compared with radium, dia-
metrically opposed extremes. Uranium is changing so
slowly that it will last for thousands of millions of years,
the emanation so rapidly that it lasts only a few weeks.
170 RADIOACTIVITY AND EVOLUTION
while radium is intermediate with a period of average
life of two thousand five hundred years.
We have seen that in many ways the emanation is
far more wonderful than radium, as the rate its energy
is given out is relatively far greater. But this is com-
pensated for by the far shorter time its activity lasts.
Also, if we compared uranium with radium, we should
say at once that radium is far more wonderful than the
uranium, whereas in reality it is not so, as the uranium,
changing almost infinitely more slowly, lasts almost
infinitely longer.
The arresting character of radium is to be ascribed
solely to the rate at which it happens to be disintegrat-
ing. The common element uranium, well known to
chemists for a century before its radioactivity was sus-
pected, is in reality even more wonderful. It is only
very feebly radioactive, and therefore is changing
excessively slowly, but it changes into radium, expelling
several «-particles and so evolving large amounts of
energy in the process. Uranium is a heavier element
than radium, and the relative weights of the two atoms,
which is a measure of their complexity, is as 238 is to
226. This bottle contains about a pound of an oxide of
uranium Avhich contains about seven-eighths of its weight
of the element uranium. In the course of the next few
thousand million years, so far as we can tell, it will
change, producing over thirteen ounces of radium, and,
in that change into radium alone, energy is given out,
as radioactive energy, aggregating of itself an enormous
total, while the radium produced will also change, giving
out a further enormous aggregate quantity of energy.
So that uranium, since it produces radium, contains
all the energy contained in a but slightly smaller quantity
of radium and more. It may be estimated that uranium
evolves during complete disintegration some thirteen
per cent, more energy than is evolved from the same
weight of radium. But what are we to say about the
other heavy elements — lead, bismuth, mercury, gold,
platinum, etc. — although their atoms are not quite so
INTERNAL ATOMIC ENERGY 171
I>eavy as uranium or radium, and although none of them,
so far as we yet know, are disintegrating at all ? Is this
enormous internal store of energy confined to the radio-
active elements, that is to the few which, however
slowly, are actually changing ? Not at all, in all
probability. Regarded merely as chemical elements
between radioactive elements and non-radioactive ele-
ments, there exists so complete a parallelism that we
cannot regard the radioactive elements as peculiar in
possessing this internal store of energy, but only as
peculiar in evolving it at a perceptible rate. Radium
especially is so completely analogous in its whole
chemical nature, and even in the character of its spec-
trum, to the non-radioactive elements, barium, stron-
tium, and calcium, that chemists at once placed radium
in the same family as these latter, and the value of its
atomic weight confirms the arrangement in the manner
required by the Periodic Law. It appears rather that
this internal store of energy we learned of for the first
time in connection with radium is possessed to greater
or lesser degree by all elements in common, and is part
and parcel of their internal structure.
The Total Energy evolved by Uranium.
Let us, however, for the sake of conciseness, leave
out of account altogether the non-radioactive elements,
of which as yet we know nothing certainly. At least
we cannot escape from the conclusion that the particular
element uranium has relatively more energy stored up
within it even than radium. Uranium is a compara-
tively common element. The world's output per year
is to be reckoned in tens of tons, whereas that of thorium,
which we have still to consider, exceeds a thousand tons.
I have already referred to the total amount of energy
evolved by radium during the course of its complete
change. It is about 360,000 times as much energy as is
evolved from the same weight of coal in burning (p. 120).
The energy evolved from uranium would be some thirteen
172 RADIOACTIVITY AND EVOLUTION
per cent, greater than from the same weight of radium.
This bottle contains about one pound of uranium oxide,
and therefore about fourteen ounces of uranium. Its
value is about £l. Is it not wonderful to reflect that in
this little bottle there lies asleep and waiting to be
evolved the energy of at least one hundred and sixty
tons of coal ? The energy in a ton of uranium would be
sufficient to light London for a year. The store of energy
in uranium would be worth a thousand times as much
as the uranium itself, if only it were under our control
and could be harnessed to do the world's work in the
same way as the energy in coal has been harnessed and '
controlled.
There is, it is true, plenty of energy in the world which
is practically valueless. The energy of the tides and of
the waste heat from steam fall into this category as
useless and low-grade energy. But the internal energy
of uranium is not of this kind. The difficulty is of
quite another character. As we have seen, we cannot
yet artificially accelerate or influence the rate of dis-
integration of an element, and therefore the energy in
uranium, which requires a thousand million years to be
evolved, is practically valueless. On the other hand,
to increase the natural rate, and to break down uranium
or any other element artificially, is simply transmuta-
tion. If we could accomplish the one so we could the
other. These two great problems, at once the oldest
and the newest in science, are one. Transmutation of
the elements carries with it the power to unlock the
internal energy of matter, and the unlocking of the
internal stores of energy in matter would, strangely
enough, be infinitely the most important and valuable
consequence of transmutation.
The Importance of Transmutation.
Let us consider in the light of present knowledge the
problem of transmutation, and see what the attempt
of the alchemist involved. To build up an ounce of a
TRANSMUTATION 173
heavy element like gold from a lighter element like
silver would require in all probability the expenditure of
the energy of some hundreds of tons of coal, so that the
ounce of gold would be dearly bought. On the other
hand, if it were possible artificially to disintegrate an
element with a heavier atom than gold and produce
gold from it, so great an amount of energy would prob-
ably be evolved that the gold in comparison would be of
little account. The energy would be far more valuable
than the gold. Although we are as ignorant as ever of
how to set about transmutation, it cannot be denied
that the knowledge recently gained constitutes a very
great help towards a proper understanding of the problem
and its ultimate accomplishment. We see clearly the
magnitude of the task and the insufficiency of even the
most powerful of the means at our disposal in a way not
before appreciated, and we have now a clear perception
of the tremendous issues at stake. Looking backwards
at the great things science has already accomplished,
and at the steady growth in power and fruitfulness of
scientific method, it can scarcely be doubted that one
day we shall come to break down and build up elements
in the laboratory as we now break down and build up
compounds, and the pulses of the world will then throb
with a new source of strength as immeasurably removed
from any we at present control as they in turn are from
the natural resources of the human savage.
Primitive Man and Fire.
It is, indeed, a strange situation we are confronted
with. The first step in the long, upward journey out
of barbarism to civilisation which man has accom-
plished appears to have been the art of kindling fire.
Those savage races who remain ignorant of this art are
regarded as on the very lowest plane. The art of kind-
ling fire is the first step towards the control and utilisa-
tion of those natural stores of energy on which civilisa-
tion even now absolutely depends. Primitive man
13
174 R^DIOACTI^TTY AKD EVOLUTION
existed entirely on the day-to-day supply of sunlight
for his \'ital energy, before he learned how to kindle fire
for himself. One can imagine before this occurred that
he became acquainted with fire and its properties from
naturally occurring conflagrations.
With reference to the newly recognised internal stores
of energy in matter we stand to-day where primitive
man first stood with regard to the energy Hberated by
fire. We are aware of its existence solely from the
naturally occurring manifestations in radioacti^'ity.
At the climax of that ci^'ihsation the first step of which
was taken in forgotten ages by primitive man, and just
when it is becoming apparent that its ever-increasing
needs cannot indefinitely be borne by the existing
supphes of energy, possibilities of an entirely new
material ci'S'ihsation are dawning with respect to which
we find ourselves still on the lowest plane — that of on-
lookers with no power to interfere. The energA^ which we
require for our very existence, and which Nature supplies
us with but grudgingly and in none too generous measure
for our needs, is in reahty locked up in immense stores
in the matter all around us, but the power to control
and use it is not yet ours. ^'^Tiat sources of energ;y" we
can and do use and control, we now regard as but the
merest leavings of Nature's primary supphes. The
very existence of the latter till now have remained un-
known and unsuspected. ^Yhen we have learned how
to transmute the elements at will the one into the other,
then, and not till then, will the key to this hidden
treasure-house of Nature be in our hands. At present
we have no hint of how even to begin the quest.
Source of Cosmical Energy.
The question has frequently been discussed whether
transmutation, so impossible to us, is not actually going
on under the transcendental conditions obtaining in the
sim and the stars. We have seen that it is actually
going on in the world under our eyes in a few special
COSMICAL ENERGY 175
cases and at a very slow rate. The possibility now
under consideration, however, is rather that it may be
going on universally or at least much more generally,
and at a much more rapid rate under celestial than
under terrestrial conditions. From the new point of
view it may be said at once that if it were so, many of
the difficulties previously experienced in accounting
for the enormous and incessant dissipation of energy
throughout the universe would disappear.
Last century has wrought a great change in scientific
thought as to the nature of the gigantic forces which
have moulded the world to its present form and which
regulated the march of events throughout the universe.
At one time it was customary to regard the evolution
of the globe as the result of a succession in the past
times of mighty cataclysms and catastrophes beside
which the eruptions of a Krakatoa or Pelee would be
insignificant. Now, however, we regard the main
process of moulding as due rather to ever-present, con-
tinuous, and irresistible actions, which, though operating
so slowly that over short periods of time their effect is
imperceptible, yet in the epochs of the cosmical calendar
effected changes so great and complete that the present
features of the globe are but a passing incident of a
continually shifting scene. Into the arena of these
silent world-creating and destroying influences and pro-
cesses has entered a new-comer — " Radioacti\dty " — and
it has not required long before it has come to be recog-
nised that in the discovery of radioactivity, or rather of
the sub' atomic powers and processes of which radio-
activity is merely the outward and -vdsible manifes-
tation, we have penetrated one of Nature's innermost
secrets.
Whether or no the processes of continuous atomic
disintegration bulk largely in the scheme of cosmical
evolution, at least it cannot be gainsaid that these pro-
cesses are at once powerful enough and slow enough to
furnish a sufficient and satisfactory explanation of the
origin of those perennial outpourings of energy by virtue
176 RADIOACTIVITY AND EVOLUTION
of which the universe to-day is a going concern rather
than a cold, Hfeless collocation of extinct worlds. Slow,
irresistible, incessant, unalterable, so apparently feeble
that it has been reserved to the generation in which we
live to discover, the processes of radioactivity, when
translated in terms of a more extended scale of space and
time, appear already as though they well may be the
ultimate controlling factors of physical evolution. For
slowprocesses of this kind do the effectivework of Nature,
and the occasional intermittent displays of Plutonic
activity correspond merely to the creaking now and
again of an otherwise silent mechanism that never stops.
Radium in the Earth's Crust.
It is one of the most pleasing features of this new
work that geologists have been among the very first to
recognise the applicability and importance of it in their
science. I am not competent to deal adequately with
or discuss the geological problems that it has raised.
But this story would be incomplete if I did not refer,
though it must be but briefly, to the labours of Pro-
fessor Strutt^ who initiated the movement and to those
of Professor Joly who has carried it on. These workers
carried out careful analyses of the representative rocks
in the earth's crust for the amount of radium they con-
tained. Absolutely, the quantity of radium in common
rocks is of course very small, although with the refined
methods now at the disposal of investigators it is quite
measurable. The important fact which has transpired,
however, is that the rocks examined contain on the
average much larger quantities of radium, and therefore
necessarily of its original parent uranium, than might
be expected. The amount of heat which finds its way
in a given time from the interior of the globe to the
surface and thence outwards into external space by
radiation has long been accurately known. Strutt
concluded that if there existed only a comparatively
1 Now Lord Rayleigh.
RADIOACTIVITY AND GEOLOGY 177
thin crust of rocks less than fifty miles thick of the same
composition, as regards the content of radium, as the
average of those he examined, the radium in them
would supply the whole of the heat lost by the globe to
outer space. He concluded that the surface rocks must
form such a thin crust, and that the interior of the globe
must be an entirely different kind of material, free from
the presence of radium. Otherwise the world would be
much hotter inside than is known to be the case. So
far then as the earth is concerned, a quantity of radium
less than in all probability actually exists would supply
all the heat lost to outer space. So that there is no
difficulty in accounting for the necessary source of heat
to maintain the existing conditions of temperature on the
earth over a period of past time as long as the uranium
which produces the radium lasts — that is to say, for a
period of thousands of millions of years.
Professor Joly in his interesting work. Radio-
activity and Geology, has considered in detail some
of the consequences of the existence of radioactive
materials in the earth. One of the specific instances is
the effect of the radium in the rocks of the Simplon
Tunnel in producing the unexpectedly high temperatures
there encountered. From a radioactive analysis of
these rocks he came to the conclusion that without undue
assumptions it is possible to explain the differences in
the temperature of the rocks encountered in boring the
tunnel by the differences in their radium content.
Various Possible Fates of the Earth.
The presence in the rock of a proportion amounting
to a few million millionths of radium above the normal
quantity very nearly wrecked the whole enterprise.
From the importance of radioactivity in this instance,
of a tunnel a few miles long bored through a mountain,
some idea may be obtained of the significance of the
new discoveries in the general problem of the thermal
condition of the interior of the globe. Since Strutt's
178 RADIOACTIVITY AND EVOLUTION
original work, it has been established that not only
radium, but all the other radioactive materials, includ-
ing the whole thorium disintegratiQn series, must con-
tribute an important quantity of heat, so that his estimate
of a crust only fifty miles thick is in reality too great,
and a much thinner crust would suffice. Joly has had
the courage to push the argument to its logical conclu-
sion, and has supposed that the radioactive materials are
not confined to a thin surface crust, but are equally
distributed throughout the globe in nmch the same
proportions as they are in the crust. If this is so, there
is no escape from the conclusion that the interior of the
earth, so far from gradually parting with its heat and
cooling down, must actually be getting steadily hotter.
The heat generated within, even after the lapse of hun-
dreds of millions of years, would scarcely appreciably
escape from the surface, for, as Lord Kelvin deduced,
the central core of the earth must be almost insulated
thermally from the surface, owingto the low conductivity
of the rocks composing the crust. He assumes through-
out an average composition of the globe of two parts of
radium per million million, which is considerably below
the average he found for the rocks of the crust, and he
calculates that in the course of a hundred million years
this minute quantity will produce a rise of the tempera-
ture of the central core of no less than 1,800° C. Unless,
therefore, this heat is utilised in some unknown way, or
the disintegration of the radio- elements is prevented by
the high temperature and pressure, the ultimate fate
of the globe must be very much as depicted in the
Biblical tradition. Sooner or later the crust must
succumb to the ever-increasing pressure within, and the
earth must become again, what it is supposed once to
have been, a vastly swollen globe of incandescent gas.
As Joly remarks, there is no evidence that this has not
already occurred more than once, nor assurance that it
will not recur. So far as physical science yet can deduce,
the accumulation of thermal energy within a world con-
taining elements undergoing atomic disintegration during
GEOLOGICAL AND INCANDESCENT AGES 179
the " geological age " must alternate with a state of
things which might be termed " the incandescent age,"
in Avhich this accumulated energy is dissipated by radia-
tion. This periodic cycle of changes must continue until
the elements in question have disintegrated — that is,
over a period which radioactive measurements indicate
is of the order of tens or hundreds of thousands of
millions of years. During the incandescent age the loss
of heat by radiation, which increases according to the
fourth power of the temperature, is immensely greater
than could be supplied even by atomic disintegration.
Thus, if the known laws hold, it is certain that the
present loss of heat of the sun cannot be supplied by
the presence of radium. For this to be the case a very
large part of the sun's 'mass must consist of uranium,
and this we know from the spectroscope is very im-
probable. Still, it is by no means to be concluded that
the heat of the sun and stars is not in the first place of
inte nal rather than, as has been the custom to regard
it, of external origin.
As soon as sufficient of the heat energy of a world
has been radiated away for a solid crust to form, the
poor thermal conductivity of this crust at once reduces
the radiation loss to a negligible figure again, a fresh
geological age is inaugurated, and again the heat accu-
mulates within. This view, that the elements contain
within themselves the energy from which Nature obtains
her primary supplies, and that in cosmical time " geo-
logical age " and " incandescent age " alternate as the
night and day, however imperfect it may still be, is at
least more in harmony with existing knowledge than the
older conventional view that the universe was wound up
once for all in the beginning like a clock to go for a
certain time, for the most part quietly and uneventfully,
pursuing its allotted path towards ultimate physical
stagnation and death. But what a picture it conjures
up of life and of the precariousness of its tenure^ — from
its lowest beginnings to its highest evolution, not a
permanent accomplishment, but a process to be inaugu-
180 RADIOACTIVITY AND EVOLUTION
rated and consummated afresh, if at all, between the
ending and beginning of each new cosmical day !
To escape from this conclusion it is necessary to
suppose that atomic disintegration is cosmically not the
inevitable uncontrollable process it has hitherto been
proved to be under all laboratory conditions, but that
under conditions of pressure and temperature, such as
exist in the interior of a world, it may either be stopped
altogether, or compensated for by unknown comple-
mentary processes of atomic synthesis in which energy
is taken up.
The Most Probable View.
The balance of probability appears to rest with the
view that the radioactivity of the materials comprising
the earth is confined to a crust and that the central core
is more or less free from radioactive matter. Our
knowledge of earthquake phenomena, and particularly
of the three distinct routes by which an earthquake wave
travels from one point on the surface of the earth to
another — (1) and (2) by circular paths clockwise and
counter-clockwise through the crust, and (3), the " P3 "
route, by a straight line joining the two points — has
strongly supported the view that the core of the earth
is of a totally different nature from the crust. On the
P3 route, once the wave gets below the crust, it travels
much faster than it does through the surface. This,
especially, confirms the picture of the earth as a metallic
sphere of nickel-steel within, surrounded with a thin
surface layer of solidified slag, which its high specific
gravity and the composition of meteorites first sug-
gested. On this view, it is to be expected that the
radioactive materials will be confined to the crust and
be absent from the metallic core, and, therefore, that
the crust may have reached a steady temperature, at
which the loss of heat by radiation is exactly balanced
by the heat evolved by its radioactive constituents. If
this is so, the present state would continue without
much change for hundreds of millions of years.
RADIOACTIVITY AND MYTHOLOGY 181
Be that as it may, our outlook on the physical uni-
verse has been permanently altered. We are no longer
the inhabitants of a universe slowly dying from the
physical exhaustion of its energy, but of a universe
which has in the internal energy of its material compo-
nents the means to rejuvenate itself perennially over
immense periods of time, intermittently and catastro-
phically, which is the first possibility that presents itself,
or continuously and in orderly fashion, if there exist
compensating phenomena still outside the ken of science.
Radioactivity and Mythology.
The world probably being of much greater antiquity
than physical science has thought to be possible, it is
interesting and harmless to speculate whether man has
shared with the world its more remote history.
In this connection it is curious how strangely some
of the old niyths and legends about matter and man
appear in the light of the recent knowledge. Consider,
for example, the ancient mystic symbol of matter,
known as Ouroboros — " the tail devourer " — which was
a serpent, coiled into a circle with the head devouring
the tail, and bearing the central motto, " The whole is
one." This symbolises evolution; moreover, it is evolu-
tion of matter — the very latest aspect of evolution — the
existence of which was strenuously denied by Clerk
Maxwell and others of only last century. The idea which
arises in one's mind as the most attractive and consistent
explanation of the universe in the light of present know-
ledge is, perhaps, that matter is breaking down and its
energy being evolved and degraded in one part of a cycle
of evolution, and in another part, still unknown to us,
the matter is being again built up with the utilisation^
of the waste energy. If one wished to symbolise such
an idea, in what better way could it be done than by the
ancient tail-devouring serpent ?
Some of the beliefs and legends which have come
down to us from antiquity are so universal and deep-
182 RADIOACTIVITY AND EVOLUTION
rooted that we are accustomed to consider them almost
as old as the race itself. One is tempted to inquire how
far the unsuspected aptness of some of these beliefs and
sayings to the point of view so recently disclosed is the
result of mere chance or coincidence, and how far it may
be evidence of a wholly unknown and unsuspected
ancient civilisation of which all other relic has dis-
appeared. It is curious to reflect, for example, upon
the remarkable legend of the philosopher's stone, one of
the oldest and most universal beliefs, the origin of which,
however far back we penetrate into the records of the
past, we do not probably trace to its real source. The
philosopher's stone was accredited the power not only
of transmuting the metals, but of acting as the elixir
f>f W^' Now, whatever the origin of this apparently
meaningless jumble of ideas may have been, it is really
a perfect and but very slightly allegorical expression of
the actual present views we hold to-day. It does not
require much effort of the imagination to see in energy
the life of the physical universe, and the key to the
primarjT^ fountains of the physical life of the universe
to-day is known to be transmutation. Is, then, this old
association of the power of transmutation with the
elixir of life merely a coincidence ? I prefer to believe
it may be an echo from one of many previous epochs in
the unrecorded history of the world, of an age of men
which have trod before the road we are treading to-day,
in a past possibly so remote that even the very atoms
of its civilisation literally have had time to disintegrate.
Let us give the imagination a moment's further free
scope in this direction, however, before closing. What
if this point of view that has now suggested itself is
true, and we may trust ourselves to the slender founda-
tion afforded by the traditions and superstitions which
have been handed down to us from a prehistoric time ?
Can we not read into them some justification for the
belief that some former forgotten race of men attained
not only to the knowledge we have so recently won, but
also to the power that is not yet ours ? Science has
THE FALL OF MAN 183
reconstructed the story of the past as one of a con-
tinuous Ascent of Man to the present-day level of his
powers. In face of the circumstantial evidence existing
of this steady upward progress of the race, the tradi-
tional view of the Fall of Man from a higher former state
has come to be more and more difficult to understand.
From our new standpoint the two points of view are by
no means so irreconcilable as they appeared. A race
which could transmute matter would have little need
to earn its bread by the sweat of its brow. If we can
judge from what our engineers accomplish with their
comparatively restricted supplies of energy, such a race
could transform a desert continent, thaw the frozen poles,
and make the whole world one smiling Garden of Eden.
Possibly they could explore the outer realms of space,
emigrating to more favourable worlds as the superfluous
to-day emigrate to more favourable continents. The
legend of the Fall of Man, possibly, may be all that has
survived of such a time before, for some unknown reason,
the whole world was plunged back again under the
undisputed sway of Nature, to begin once more its
upward toilsome journey through the ages.
The New Prospect.
The vistas of new thought which have opened out in
all directions in the physical sciences, to which man is
merely incidental and external, must in turn react
powerfully upon those departments of thought in which
man is central and supreme. We find ourselves in con-
sequence of the progress of physical science at the pin-
nacle of one ascent of civilisation, taking the first step
upwards out on to the lowest plane of the next. Above
us still rises indefinitely the ascent to physical power —
far beyond the dreams of mortals in any previous system
of philosophy. These possibilities of a newer order of
things, of a more exalted material destiny than any
which have been foretold, are not the promise of another
world. They exist in this, to be fought and struggled
184 RADIOACTIVITY AND EVOLUTION
for in the old familiar way, to be wrung from the grip of
Nature, as all our achievements and civilisation have,
in the past, been wrung by the labour of the collective
brain of mankind guiding, directing, and multiplying
the individual's puny power. This is the message of
hope and inspiration to the race which radium has con-
tributed to the great problems of existence. No attempt
at presentation of this new subject could be considered
complete which did not, however imperfectly, suggest
something of this side. It is fitting to attempt to see
how far purely physical considerations will take us in
delimiting the major controlling influences which regu-
late our existence.
Surveying the long chequered, but on the whole con-
tinuous, ascent of man from primeval conditions to the
summit of his present-day powers, what has it all been at
bottom but a fight with Nature for energy — for that
ordinary physical energy of which we have said so much ?
Physical science sums up accurately in that one generali-
sation the most fundamental aspect of life in the sense
already defined.
Of course life depends also on a continual supply of
matter as well as on a continual supply of energy, but
the struggle for physical energy is probably the more
fundamental and general aspect of existence in all its
forms. The same matter, the same chemical elements,
serve the purposes of life over and over again, but the
supply of fresh energy must be continuous. By the law
of the availability of energy, which, whether universal
or not, applies universally within our own experience,
the transformations of energy which occur in Nature
are invariably in the one direction, the more available
forms passing into the waste and useless unavailable
kind, and this process, so far as we yet know, is never
reversed. The same energy is available but once. The
struggle for existence is at the bottom a continuous
struggle for fresh physical energy.
This is as far as the knowledge available last century
went. What is now the case ? The aboriginal savage,
THE NEW PROSPECT 185
ignorant of agriculture and of the means of kindling fire,
perished from cold and hunger unless he subsisted as a
beast of prey and succeeded in plundering and devouring
other animals. Although the potentialities of warmth
and food existed all round him, and must have been
known to him from natural processes, he knew not yet
how to use them for his own purposes. It is much the
same to-day. With all our civilisation, we still subsist,
struggling among ourselves for a sufficiency of the
limited supply of physical energy available, while all
around are vast potentialities of the means of susten-
ance, we know of from naturally occurring processes, but
do not yet know how to use or control. Radium has
taught us that there is no limit to the amount of energy
in the world available to support life, save only the limit
imposed by the boundaries of knowledge.
It cannot be denied that, so far as the future is con-
cerned, an entirely new prospect has been opened up.
By these achievements of experimental science Man's
inheritance has increased, his aspirations have been up-
lifted, and his destiny has been ennobled to an extent
beyond our present power to foretell. The real wealth
of the world is its energy, and by these discoveries it,
for the first time, transpires that the hard struggle for
existence on the bare leavings of natural energy in which
the race has evolved is no longer the only possible or
enduring lot of Man. It is a legitimate aspiration to
believe that one day he will attain the power to regulate
for his own purposes the primary fountains of energy
which Nature now so jealously conserves for the future.
The fulfilment of this aspiration is, no doubt, far off,
but the possibility alters somewhat the relation of Man
to his environment, and adds a dignity of its own to
the actualities of existence.
PART II
CHAPTER XII
THE THORIUM AND ACTINIUM DIS-
INTEGRATION SERIES
The Thorium Disintegeation Series.
Those who have mastered the intricacies of the uranium
disintegration series may wish to know something of the
important developments which have taken place since
these lectures were first given in 1908, and of the other
two great disintegration series known to science, the
thorium and the actinium series. Space precludes a
description as detailed and non-technical as that before
aimed at, and in some of the more difficult sections it
will be necessary to assume a considerable knowledge
on the part of the reader of physical and chemical
science. But the attempt seems worth making for the
sake of completeness.
The thorium disintegration series is becoming in-
creasingly important, and its consideration does not
involve any new principles. Thorium is an element
which was at one time rare and little known even to
chemists, but has come into prominence during the last
twenty years, because of its use as a constituent of the
incandescent gas-mantle, which is composed of about
99 per cent, of thorium oxide, and 1 per cent, of cerium
oxide. Fairly abundant sources of thorium have been
discovered in the sands of certain coasts in Brazil,
North and South Carolina, etc., where a natural con-
centration has taken place by the action of the sea-
waves of the particles of the heavy mineral monazite,
186
THORIUM 187
which occurs as a minute constituent in many rocks,
and in the sands derived from them by the action of
weathering agencies. The monazite is concentrated
from the sand usually by magnetic methods, until it
contains 4 per cent, of thorium oxide. This constitutes
the monazite sand of commerce, and from it every year
hundreds of tons of pure thorium salts are now manu-
factured for the gas-mantle industry. More recently the
find has been made of a very rich monazite in Central
India, containing nearly 10 per cent, of thorium.
Mesothorium and Radiothorium.
As already described, the usual a-radioactivity of
commercial thorium compounds is of about the same
strength as that of pure uranium compounds, but the
/3- and 7-, or penetrating activity, is several times less
intense. We have seen (p. 153) that in the uranium
minerals, although several intermediate products of the
disintegration of uranium are present, with periods of
life sufficiently long, and radioactivity sufficiently im-
portant, to repay extraction, it is practicable to extract
only one of these — namely, radium. In the thorium
minerals there are two such products, named meso-
thorium and radiothorium, and though their periods of
average life, about eight years and three years respec-
tively, are very much less than that of radium, they are
sufficiently long to make their extraction and utilisation
practicable. Whereas the sources of radium are costly
and comparatively limited in amount, the by-products
of the thorium industry, after the extraction of the
technically valuable thorium, are the source from which
mesothorium and radiothorium are extracted. Much
greater quantities of these by-products have to be
handled, it is true, than in the extraction even of radium
from pitchblende to produce similar results. The new
substances must, on this account, always be costly to
produce. But in the by-products of a single year's
manufacture of thorium the new products capable of
188 THORIUM AND ACTINIUM
being extracted possess as much radioactivity as at
least an ounce of pure radium. They thus offer an
abundant source of radioactive material, which at present
is mostly wasted, and the product, while it lasts, is in
every respect the equal of radium in its properties. The
only disadvantage it possesses is its relatively much
shorter period of life.
The discoveries in the thorium series of these two
technically valuable members were made by Otto Hahn,
who has worked both with Sir William Ramsay and Sir
Ernest Rutherford, comparatively recently, after the
rest of the members had become quite well known. The
historical development of the subject from the first dis-
covery of the radioactivity of thorium compounds up
to the present time is a most interesting chapter to the
student, but would unduly complicate the subject if
considered here. It is better to proceed in order through
the thorium disintegration series as it is at present
known, apart from historical considerations as to the
order in which they were discovered, though, as in
the case of the uranium series, the first members were
the last to be separately recognised.
Radioactivity of Thorium.
Unlike pure uranium salts, which, a few months after
preparation, have a definite constant radioactivity,
consisting of all three types of rays, the a-activity being
due to uranium, and the 13- and 7-activity to the short-
lived uranium X in equilibrium with it, thorium salts,
though chemically pure, vary continuously in their whole
radioactivity for twenty or thirty years after manu-
facture. Even after these periods, slight changes must
still be going on, and probably fifty years would have
to elapse before they became quite inappreciable. But
in spite of the great apparent differences between the two
elements, there is a very close analogy in their disin-
tegration series, every one of the eleven known members
of the thorium series having an analogue in the twelve
RADIOACTIVITY OF THORIUM 189
members of the uranium series as far as radium D, at
which point the thorium disintegration appears to come
to an end. One a.-ray giving product in the uranium
series is not represented in the thorium series. The
analogous members in the two series usually give out
similar kinds of rays, and although their periods are
often widely different, there is a rough correspondence in
the two series between the relative periods of the suc-
cessive members, the periods in the thorium series being,
however, usually much less than in the uranium series.
Thus uranium I, with its period of 8,000,000 years, gives
a-rays, and is followed by uranium X^, giving (/3)-rays,
of period 35-5 days, and by uranium X2, or brevium,
of very short period, which gives powerful and penetrat-
ing /S-rays. Uranium II,' which follows, is chemically
identical with uranium I, and, like it, is of long period
and gives a-rays. This produces ionium, which gives
«-rays, and has the period of 100,000 years. Ionium, in
turn, produces radium, which gives a-rays, and has a
period of 2,500 years. Thorium itself is provisionally
estimated to have a period about three times longer than
uranium I, and gives only a-rays. It produces by its
disintegration " mesothorium I," which does not give
any important rays, and has a period of 7-9 years. It
is identical in chemical character with radium, and
corresponds with uranium X^, except that no /S-rays at
all are expelled. Its product is called " mesothorium
II," which corresponds very closely with uranium Xg,
giving out powerful yS- and 7-rays, and having a period
of only 8-9 hours. It produces in turn " radiothorium, "
which corresponds perfectly with ionium, giving a-rays,
and having a period of 2-9 years. These last two sub-
stances are chemically identical with one another, and
also with thorium itself, and cannot be separated by any
known method when mixed together. This fact is of
considerable importance, as thorium when separated
from a mineral, always contains at first all the radio-
thorium in the mineral and also all the ionium, if ura-
nium was also present, as is almost invariably the case.
14
190 THORIUM AND ACTINIUM
The product of radiothorium is thorium X, which corre-
sponds with radium, giving a-rays, but having a period
of only 5-6 days. Thorium X is chemically identical
with radium, and also with mesothorium I. This
chemical identity of radium and mesothorium I is the
dominating fact in the separation of these new sub-
stances, as will later be more clear. After thorium X,
the thorium emanation results, corresponding perfectly
in its whole nature as a member of the argon family of
gases, with the radium emanation, and giving a-rays,
but having the much shorter period of only 76 seconds.
Its product is the thorium active deposit, of which the
Meso- Meao- Radio- Thorium X.
thorium I. thorium II. thorium.
25,000,000,000 7-9 years. 8-9 hours. 2-91 years. 5-35 days.
(?) years.
Emanation. Thorium A. Thorium B. Thorium C, Thorium D. Thorium E.
76 seconds. 0-2 second. 16'3 hours. 79 minutes. 4-5 minutes. (Lead.)
Fig. 37.
first three members, called thorium A, B, C, are almost
precisely analogous to the corresponding radium mem-
bers, except in period. The period of thorium A is only
one-fifth of a second. Those of thorium B and C are
15-3 hours and 79 minutes respectively. These last are
the only two, except thorium itself, possessing periods
longer than the corresponding members of the uranium
series. Lastly, there exists, as the product of thorium
C, thorium D, the last active member known, which gives
^- and 7-rays, and has the short period of 4-5 minutes.
It has little analogy to radium D. The ultimate pro-
duct of thorium was till recently not even guessed. All
that could be said is that its atomic weight, calculated
THE THORIUM SERIES 191
from that of thorium and the number of a-particles ex-
pelled, is 208, and this is the atomic weight of bismuth !
It cannot be bismuth, because in some ancient thorium
minerals hardly a trace of bismuth can be found. The
discovery of its nature came as a surprise, for in spite of
the difference of atomic weight, it proves to be the same
element as ends the uranium series — namely, lead. This
has raised very deep issues. The complete thorium
disintegration series is shown on p. 190 (Fig. 37), so far as
we have yet considered it. But thorium C, there shown
single, is like radium C complex (see p. 201).
The extraordinary analogies between this series and
the uranium series, on the one hand, and the actinium
series, on the other, will later receive a very satisfying
explanation.
Mesothorium.
It is clear that mesothorium I, with the period of
average life of nearly eight years, being both the first
and the longest lived of the successive products, is the
centre of interest. The radioactivity of the element
thorium itself, consisting only of low-range a-rays, of
relatively feeble intensity because of the enormous period
of the element, is technically and scientifically even of
less interest than that of uranium. Mesothorium, how-
ever, corresponds to radium in that it can be concen-
trated, and the greater part of the radioactivity of many
tons of minerals can be separated in a preparation weigh-
ing less than a few milligrams. Just as when radium is
first prepared it gives only the relatively unimportant
a-activity proper to itself, but in course of time develops
enormously in all its activity due to the growth and
accumulation of its products, so it is with mesothorium.
Freshly prepared and free from its products, it has
practically no activity. In the course of a few hours
the strong penetrating activity of its short-lived product,
mesothorium II, develops, and in two or three days this
reaches a maximum or equilibrium value. This part of
the activity then remains, so long as the preparation is
192 THORIUM AND ACTINIUM
not chemically treated, apparently constant, but actually
decaying very slowly. This decay is to half the initial
value after 5-5 years, to a quarter after 11 years, and so on.
But the product of this change is radiothorium, which
gives cc-rays; and, just as in the case of radium, this is
followed by a small host of short-lived products, some of
which give a- and others /3- and 7-rays. What actually
happens, therefore, is that in addition to the initial
rapid growth of /S- and 7-rays already discussed, a slow
steady increase of the a-, /3-, and 7-activity of a meso-
thorium preparation takes place for many years after its
preparation, due to the growth and accumulation of
radiothorium and its products. It is calculated that
this increase will go on for about four and a half years,
and then the activity of the preparation will reach a
maximum, the penetrating activity (yS- and 7-rays)
being then nearly twice that at two days after prepara-
tion. From then onwards the regular slow decay of all
the radioactivity will set in, and continue with the half-
period of five and a half years, as already considered.
Twenty years after preparation the activity will be
some 12 per cent., whilst after a century it would be less
than 1,000th per cent, of the maximum activity.
In practice, however, the change is even more com-
plicated than this on account of the invariable presence
of radium in the mesothorium preparations. Radium
and mesothorium form, as already remarked, an example
of which now so many exist in radioactivity, of two
different elements, having entirely different radioactive,
but entirely identical, chemical character. For a long
time the nature of the chemical processes used to extract
mesothorium from the by-products of monazite was kept
secret. It was thought that they were peculiarly diffi-
cult and forbidding. I was therefore surprised and
interested to find — and the same discovery was made
at about the same time by Professor Marckwald in Berlin
— that mesothorium and radium behaved in chemical
processes identically. In consequence the extraction
of mesothorium from monazite residues is entirely
MESOTHORIUM 193
similar in principle to that of radium from pitchblende
residues. Since monazite always contains a minute
amount of uranium, and therefore the corresponding
quantity of radium, the mesothorium separated always
contains the radium also. No successful separation has
as yet been achieved, and it is most improbable that it
ever will be. After a lengthy fractional crystallisation
of the mixture I found the relative proportions of the
two elements entirely unaltered. Technical meso-
thorium owes about 12 per cent, of what has been
termed its maximum activity (that after four and a half
years) to radium. This activity will remain when all
that due to mesothorium has completely decayed away.
In practice, therefore, the decay of the preparations will
be appreciably slower than if radium were absent.
These discoveries have thus resulted in the provision
of an effective substitute for radium, which for such
purposes as medical application, or for general researches
in the properties of the new radiations, are, while the
activity lasts, its equal in every respect. Indeed, it is
possible to obtain mesothorium preparations many
times more concentrated in their activity than pure
radium salts. There is no dearth of the raw material,
which hitherto has been a wasted product.
But, of course, from the strictly scientific point of
view, the radioactivity of mesothorium is as distinct
from that of radium as copper is from zinc, or as one
flower is from another. It will be of interest to con-
centrate upon some of the chief resemblances and differ-
ences in the two disintegration series.
The Thorium Emanation.
The thorium emanation was the first of the three
emanations to be discovered, and had been fairly com-
pletely investigated by Rutherford before the others
were known. It is given off in greater or less degree by
all thorium compounds. If the radioactivity of the
compound is measured by placing it in a closed electro-
194 THORIUM AND ACTINIUM
scope, the activity is found to increase for about ten
minutes, owing to the accumulation of the emanation,
and then remains constant if the instrument is not dis-
turbed. But if a current of air is blown through the
instrument, it sweeps out the emanation, and the acti-
vity is correspondingly reduced. On stopping the blast
of air, it rises again precisely as at first. Uranium com-
pounds show no trace of this behaviour, as they do not
generate an emanation. The products of the disintegra-
tion of the thorium emanation are known as the thorium
active deposit, and they manifest themselves in much the
same way as the radium active deposit, being attracted
to the negatively charged surface in an electric field.
They last much longer, however, the period of half-
decay being about eleven hours instead of about half an
hour, and, in consequence, they take longer to accu-
mulate. In a vessel containing a thorium or, better, a
radiothorium preparation, which acts as a constant
source of the evanescent thorium emanation, the active
deposit on the walls of the vessel (or on the negative
electrode, if an electric field is used), goes on increasing
in amount for about two days, whereas in the radium
emanation the active deposit reaches the maximum
value in about three hours.
Radiothorium.
Radiothorium is the most powerful and convenient
source of the emanation and active deposit of thorium.
As already explained, radiothorium is not separable
from thorium by any chemical process. Freshly pre-
pared thorium compounds contain practically all the
radiothorium of the original mineral, but its parent meso-
thorium being absent, this radiothorium in the course
of a few years decays. Before it decays completely,
however, mesothorium has been regenerated by the
thorium, and in time begins to produce fresh radio-
thorium. The consequence is that commercial thorium
compounds always contain more or less radiothorium,
RADIOTHORIUM 195
and always, therefore, furnish more or less of the emana-
tion and active deposit. But the amount is insignificant
compared with what can now be obtained from a com-
mercial radiothorium preparation. Mesothorium, after
it is separated from the mineral and left to itself, pro-
duces, as we have seen, radiothorium. After a year or
more of accumulation these two substances may with
advantage be separated. A trace of a thorium salt is
added to the solution, and then precipitated by adding
ammonia as thorium hydroxide, which carries with it the
whole of tha radiothorium, leaving the mesothorium in
solution. This radiothorium preparation in turn gener-
ates thorium X, and after a few weeks becomes a power-
ful source of the thorium emanation during the few years
it lasts.
Apart from the intrinsic interest attaching to this
method of " growing " radio-elements otherwise not
separable from the raw material a point of great
philosophical interest is involved. Were it not for the
existence of mesothorium intermediate between and
chemically distinct from thorium and radiothorium, the
separate existence of the latter might not have been
suspected, and they certainly could never be obtained
as individuals. In the case of uranium I and uranium
II, the evidence for the existence of two elements remains
indirect, and they have never yet been separated. The
intervening member, uranium X, is too short-lived and
the product uranium II too long-lived for the quantity
of the latter produced from the former to be detectable
even by radioactive methods (vide pp. 129 and 150). One
can hardly help wondering how many of the well-known
common so-called elements may not be mixtures of more
than one element with chemically identical properties.
Experiments with the Thorium Emanation.
Radiothorium may be used to show, in a striking way,
by means of phosphorescent screens, many of the older
classical experiments on the growth and decay of radio-
196 THORIUM AND ACTINIUM
active substances on which the existing theory of atomic
disintegration has been built up. For example, if a
radiothorium preparation or old mesothorium prepara-
tion containing radiothorium, is kept in a tube through
which a puff of air can be sent from a rubber blower,
and the accumulated emanation is thus blown out into
a flask internally coated with zinc sulphide, as shown in
Figs. 8 and 9, it will cause it to phosphoresce briJliantly
in the dark. The decay of the emanation in the flask
can then be watched from minute to minute with the
eyes, and its concomitant reproduction in the radio-
thorium tube can easily be demonstrated. For example,
the radiothorium tube may first be thoroughly blown
out, and then the effect observed of blowing through it
into a zinc sulphide flask immediately, before any emana
tion has had time to accumulate, and then after waiting
successive periods of, say, ten, twenty, thirty seconds,
one, two, ten, or more minutes. For the shorter intervals
the amount of emanation produced is very nearly pro-
portional to the time, but for the longer ones the decay
of that produced first during the period of accumulation
begins to tell, and the increase with time gets less and
less. So that after five or ten minutes no increase
results, however long a time is allowed to elapse. The
emanation is then in " equilibrium," as much decaying
per second as is produced per second.
In this way many of the simple laws of the decay and
reproduction of the emanation, on which the whole super-
structure of radioactive theory was at first largely based,
may now be shown to a large audience. But all the
original work was done with delicate electrical instru-
ments long before anyone had ever observed a single
visible effect, or had any other than indirect elec-
trical evidence of the existence of the evanescent
emanation.
THORIUM A 197
Thorium A.
The most recent member to be added to the thorium
disintegration series is thorium A, the direct product
of the emanation, which, on account of its short period
of average Hfe — about one-fifth of a second only — had
hitherto not been separately distinguished from the
emanation. It was put in evidence by Rutherford and
his colleagues in the following ingenious manner: An
endless wire passed along the axis of a cylinder, con-
taining a radiothorium preparation, through holes in
ebonite stoppers closing the ends of the cylinder, and
over suitable pulleys outside of the cylinder driven by
an electric motor. In this way the wire was kept passing
through a cylinder filled with thorium emanation. The
wire was connected to the negative and the cylinder to
the positive pole of a battery, so as to concentrate the
active deposit on the wire. It was found that the wire
immediately after leaving the cylinder was intensely
active, giving out powerful a-rays, and capable of light-
ing up a zinc sulphide screen brought near to the wire.
This activity on the wire lasts only a small fraction of a
second, so that after the wire has moved away a little
from the cylinder its activity has practically disappeared.
Thus, although the wire is being driven at a high speed
all the time, it is only the part immediately issuing from
the cylinder which is active, and which causes the sul-
phide screen to glow. Thorium A is a non-volatile
product of the gaseous emanation, and is attracted to
the negative electrode. But almost as soon as it is
deposited it breaks up, giving «-rays. On the principle
of a short life and a merry one, the effect of this product
is far more marked, for short periods of exposure, than
that of the longer-lived products it in turn produces.
Though it would be easy to show that the wire, after the
large activity due to thorium A is over, still possesses
activity due to the products formed, this activity is,
for short periods of exposure, far too small to light up a
198 THORIUM AND ACTINIUM
phosphorescent screen. In this way the existence of
this almost hopelessly unstable element has been demon-
strated. In connection with the thorium active deposit
and the complex character of thorium C something has
still to be said, but it may be deferred.
The Actinium Disintegration Series.
A few words may be said for the sake of completeness
on the third and least important disintegration series,
but one which is, however, just as interesting to the
student as the others. In addition to the polonium and
radium separated from pitchblende by M. and Mme.
Curie, a colleague, M. Debierne, was successful in isolat-
ing a third new radio-element, to which he gave the
name Actinium. So far as is known, actinium is at
least a fairly long-lived radio-element, for although it
was discovered very shortly after radium, the original
preparations have retained much, at least, of their
activity. Recently it has been established that a slow
decay, however, does occur which indicates a period of
average life for this substance of only about thirty years.
Actinium is separated with the " rare earths " in
uranium minerals, and chemically it resembles most
closely the rare-earth element, lanthanum, although it
is not completely identical with it in chemical pro-
perties. In radioactive properties its disintegration
series is very closely analogous to that of thorium, and
consists of eight members, in addition to itself, the first
of which, known as radioactinium, corresponds with
radiothorium. The next is actinium X, corresponding
perfectly with thorium X, and after tPiat the actinium
emanation, actinium A, B, C, and D, follow in regular
order, almost exactly as in the thorium series. The
analogous products in the two series in each case give
out the same kinds of rays, and are, so far as is known,
chemically identical in character. But, almost without
exception, the periods in the actinium series are uni-
formly shorter than in the thorium series, the longest,
ACTINIUM 199
that of radioactinium, being only twenty-eight days,
and the shortest, that of actinium A, being only ^oth
of a second. The full series is shown in Fig. 38.
O^ D^ O^ O^
oo-ck>ch
Actinium. Badio- Actinium X. Emanation. Actinium A.
actinium.
SOyeai-s. 28-1 days. 15 days. 5-6 seconds. 0'003 second
Actinium Actinium Actinium Actinium
B. C. D. E.
52-1 minutes. 3'1 minutes. 6*83 minutes, (unknown).
Fig. 38.
The Origin of Actinium.
Whereas it is customary to regard the uranium and
thorium series each as starting from a primary parent of
so long Ufe, that, old as the world is, some still survives
unchanged, the problems connected with the real nature
and origin of actinium are still not entirely cleared up.
Its short period of life, recently established, proves that
it cannot itself be a primary radio-element like the
other two, and, in fact, its parent is now known. But
it is not impossible that it may form part of a third
independent primary series, though this has not been
the view that has so far gained most support. So far as
knowledge has been gained, actinium appears to be found
only in the uranium minerals and in all of these which
have been examined for it. It is natural to conclude
from this that it is a product of uranium. But here a
difficulty arises. In the disintegration of actinium at
least five a-particles are given out per atom disintegrat-
ing, representing a loss in atomic weight of 20 units.
There is certainly no room for the actinium series between
uranium and polonium, and there is no evidence that it
comes after polonium.
200 THORIUM AND ACTINIUM
Multiple Atomic Disintegration.
The important piece of evidence, however, which
shows conclusively that actinium cannot be in the main
uranium-polonium series, and which at the same time
serves to distinguish this series from the others, and to
make it practically the most difficult to investigate, is
the extraordinarily small relative quantity of actinium
in uranium minerals. Although the actinium series
gives out at least five a-particles per atom as compared
with eight given out by the uranium-radium-polonium
series, the a-radiation contributed by the whole actinium
series in uranium minerals is only about one-fifteenth or
one-sixteenth of that contributed by the uranium series.
Whereas, if actinium were in the main line of descent
from uranium, the a-activity of its series should be of
the order of five-eights of that of the uranium series, in
accordance with the principle discussed on p. 153. Two
possibilities may be advanced. Either actinium is an
entirely separate and independent primary radio-
element, and, if so, its occurrence always in uranium
minerals, and only in those minerals, is difficult to
understand; or actinium may be derived from uranium
as a branch, or offshoot, not in the main line of descent.
One may suppose that at some stage of the disintegra-
tion of the uranium atom a choice of two modes of dis-
integration presents itself. The large majority of the
atoms choose one way — the way leading to polonium —
whilst a small minority choose a second way, the way
leading through the -actinium series. If this is true, it
can be calculated that out of every twelve uranium
atoms, eleven go through the main line of descent to-
wards polonium, and one goes through the actinium line.
This mode of explaining actinium is now supported by
much new evidence and by the discovery very recently
of actual cases of such a multiple disintegration at
the ends of the thorium and radium series, among the
active deposit products.
BRANCH SERIES 201
Branch Series of Thorium and Radium.
This may now be briefly dealt with. We have already
considered the evidence (p. 164) for supposing that, on
account of the very high speed at which the a-particles
are expelled from radium C and thorium C, the changes
of these substances are complex, and that the a-rays in
each case probably result from subsequent products,
named radium C and thorium C, of excessively short
life-period, which is estimated to be one millionth of a
second in the case of the former and one hundred
thousand millionth of a second in the case of the latter.
In addition, the changes are complicated by branchings
of the kind just considered, but especially instructive.
Taking the case of thorium C first, it is known that it
breaks up in two ways. In the first mode an a-ray is
expelled and the product formed then expels a yS-ray.
In the second mode the order is reversed, the /3-ray
being expelled first and the a-ray second. This may
be represented (see Fig. 39).
Range 4-55 cm.
/ ^^
O Range 8-i6cm.
a.
©— o
121-5 min. io~"'secs.
Branching of the Thorium Series.
Fig. 39.
About 35 per cent, of the atoms disintegrating follow
the first mode producing thorium D, and give out a-rays
of range 4*55 cm. ; whereas 65 per cent, give out /3-rays
in the second mode producing the hypothetical and
hopelessly evanescent thorium C, which gives out
202 THORIUM AND ACTINIUM
a-rays of range 8-16 cm. The two end products are
of the same atomic weight, 208, and whether or not
they are really identical cannot yet be said. The two
separate periods of average life for thorium C shown in
the figure, 225-7 and 121-5 minutes, are those calculated
for the two kinds of change separately, assuming that
the other did not occur.
In the case of radium C, the same applies with the
exception that only 0-03 per cent, of the atoms follow
the " «- then jS-mode," the overwhelming preponder-
ance, 99-97 per cent., following the " /3- then a-mode "
(see Fig. 40).
10"^ sees. z4 years
Etc.
Q y ") Calculated range
28.1mm. Kja #^ '^
V /
6.5 days 1-9 min.
Branching of the Radium Series
Fig. 40.
The product produced in the first mode, called
radium Cg is in such infinitesimal quantity, that little
is known about it beyond the value of its period and the
fact that it gives /3-rays.
The Actinium Branch Series.
Reverting now to actinium, the practical consequence
of its being formed only in the minor mode of a dual
disintegration, claiming only some 8 per cent, of the
uranium atoms disintegrating, is that the substance is
very much rarer and more difficult to obtain than the
members of either of the other two series. If it were
more common, it would lend itself to many demonstra-
THE ACTINIUM EMANATION 203
tions and experiments similar to those detailed under
radiothorium, but of an even more striking character.
Actinium is relatively poor in penetrating rays, and even
the most active preparations it is possible to procure
are disappointing in this respect when compared with
radium.
The Actinium Emanation.
The chief glory of actinium, however, is its emanation,
a gaseous disintegration product, precisely analogous
in every respect to those of radium and of thorium, but
having a period of average life of only 5-6 seconds. The
usual principle of a short life and a merry one applies.
The dominating characteristic of the radioactivity of
actinium preparations is the emanation that is given off.
In the dark room, if a preparation is held over a zinc
sulphide screen, the emanation diffusing away lights up
the screen in patches, which are wafted from one part
of the screen to another by draughts or any gentle puffs
of air. The rapid decay of the emanation and corre-
sponding rapid regeneration from the actinium prepara-
tion makes it quite possible to experiment thus with
the emanation in the open-air of the room. Whereas if
the radium emanation were dealt with in this way, once
it had been dissipated throughout the air of a room,
some weeks would have to elapse before a fresh supply
was available. Giesel, who rediscovered the substance
subsequently to Debierne, actually named it " Ema-
nium " before it was found to be identical with actinium.
Actinium A.
The only other product of actinium which calls for
special mention is actinium A, the direct product of the
emanation, which, like thorium A, has an extraordinarily
short period of life. Indeed, actinium A is the most
unstable element directly known, its period being only
about ^-girth of a second. It may be put into evidence
by the same device as that described for thorium A
(p. 197), but, of course, the endless wire has to be driven
considerably more rapidly than is necessary to exhibit
204 THORIUM AND ACTINIUM
the thorium product. As a matter of fact, a forgotten
experiment of Giesel, eight years before the discovery
of actinium A, clearly puts the existence of that sub-
stance into evidence, when rightly interpreted. If a
zinc sulphide screen is brought opposite to the open end
of a tube containing an actinium preparation, and a
little distance away from it, there is a diffuse luminosity
produced on the screen in the dark, due to the emana-
tion escaping from the tube. If the screen is now con-
nected with the negative pole of an electrical machine,
instantly there flashes out on the screen a sharply-defined
bright spot of the same geometrical form as the opening
of the tube. On discharging the screen this spot in-
stantly disappears. Giesel thought, very naturally,
that he was dealing with a new kind of radiation,
attracted by a negatively charged surface, and called
the supposed radiation the " E-ray," in brief for " emana-
tion ray." However, it is not the ray, but the exces-
sively short-lived product giving an ordinary «-ray,
which is attracted to the negative surface; but owing to
the infinitesimal time this product remains in existence
it appears as if it is the ray, rather than the product,
which is attracted by the electric field. Another way
of showing the same experiment is to coat a wire with
zinc sulphide, and to immerse it in a flask containing
an actinium preparation. On charging the wire nega-
tively to the flask, the zinc sulphide instantly flashes
out and remains brilliantly luminous ; but on discharging
the wire, the luminosity disappears apparently instan-
taneously. The same device can be used to show the
existence of thorium A, but an appreciable, though
small, time-lag occurs before the appearance and the
decay of the luminosity.
Eka-tantalum or Proto-actinium.
In 1919 the main problem of the origin of actinium
was cleared up by the discovery and isolation of its
direct parent in uranium minerals by Cranston and the
THE PARENT OF ACTINIUM 205
writer in this country, who named it "eka-tantahim,"
and by Otto Hahn and Miss Meitner in Germany, who
named it "proto-actinium." In each case the search
was helped by some wide and far-reaching generahsa-
tions, still to be dealt with, from which it was possible
to predict the chemical character of the missing parent
and its place in the periodic table. This place was the
last and still vacant place in the niobium-tantalum
family, between uranium on the one side and thorium
on the other. Mendelejeff, who was one of the dis-
coverers of the Periodic Law, had called attention to
three vacant places in the families of boron, aluminium,
and silicon respectively, which he assumed were occupied
by three elements still to be discovered, and which he
called eka-boron, eka-aluminium and eka- silicon. In
each case he was bold enough to predict their chemical
character from their position in the table. Shortly
afterwards the three missing elements were found, and
named scandium, gaUium, and germanium, and their
properties were found to correspond very closely with
what had been predicted.
In the present case " eka- tantalum," a still unknown
element, analogous in chemical character to tantalum,
had been foreseen to be probably the missing parent of
actinium. Beyond the fact that it has been isolated
and that it produces actinium slowly and steadily with
the lapse of time, just as ionium produces radium, not
much work has yet been done on it. It gives a-rays,
and from the range of these it is estimated that its period
is of the order of from ten to a hundred thousand years.
Uranium Y.
One more member remains to be considered, and that
is uranium Y, a radioactive product of short period of
average life, 2*2 days, discovered by Antonoff in 1911
to be produced by uranium, and giving (/3)-rays some-
what more penetrating than those of uranium Xj. It
is probable that this is the immediate parent of eka-
15
206 THORIUM AND ACTINIUM
tantalum, and the first member of the actinium branch
series. The branching is thought to occur either at
uranium I or uranium II, probably the latter, and that
in both branches an a-iay is expelled. So that
the initial changes of the series, represented in Fig. 28
as a single change, has been gradually and with diffi-
culty traced out to be something as follows:
^® ■^•(^^ ^•;3r«"rf7; ® © @
(238) ^^34) ^ (234) ^(234) &-(23o) ^(226) >- &c.
Uranium I Uranium X, Uranium X2 Uranium II Ionium Radium
8,000,000,000 3S-S days 1-65 minutes 3,ooo,ooo\ 100,000 years 2,500 years
years yea.rs(?) >l^^»(W._^^®
^230^ — ^(230) -^^26) — ^ &C.
Uranium Y Eka Tantalum Actinium
2-3 days 10,000 to 30 years (?)
100,000 years (?)
Fig. 41.
Considerations to be now dealt with have shown it to
be of extreme importance that every change in the series
should be separately and correctly recognised, and when
this was sufficiently the case, a very great advance
indeed resulted.
Thus, with the discovery of these remaining sub-
stances, the science of radioactivity now embraces
thirty-six examples of elements in the course of spon-
taneous change, with periods varying from tens of
thousands of millions of years on the one hand, to a
few billionths of a second on the other. It is unlikely
that any more of these unstable elements remain to be
discovered, unless some entirely unknown and un-
suspected source of radioactive materials is found. The
complete series are set out in detail in the table opposite
(Fig. 42).
The Unsolved Riddle of Matter.
There remains unsolved that most fundamental and
inaccessible problem, which at the same time appears
to be the problem of ultimately the most practical signi-
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208 THORIUM AND ACTINIUM
ficance — the real internal nature of matter. How is the
atom of matter put together, and how can it be pulled
apart ? These are the practical questions which the
discoveries of radioactivity raise in a pressing form
without as yet affording a hint of the answer. But in
spite of that, our knowledge of the internal structure
of the atom continues to grow at a very rapid rate, and
some of this more recent work may now be dealt with.
CHAPTER XIII
THE ULTIMATE STRUCTURE OF MATTER
A Flood of Knowledge.
Ever since the recognition of radioactivity, the dis-
covery of radium, the establishment of the theory of
atomic disintegration, and the independent proof by the
spectroscope that the element helium is actually being
produced in a natural transmutation — discoveries
which followed one another rapidly as the nineteenth
century passed away and the present century succeeded
it — it must have seemed to many that such a period of
pioneering and fundamental reconstruction in science
would soon exhaust itself and be succeeded by one of
steady spade-work in the cultivation of the new terri-
tory opened up. The development of the new territory
and its detailed exploration have gone on steadily and
rapidly, but, so far from the wave of original and fertile
ideas having exhausted itself, the initial successes above
mentioned have proved to be but the first indications
of a continuously advancing tide. Already from many
totally distinct directions the flood of knowledge has
revealed many of the deeper secrets of the constitution
of matter. Ignorance and impotence in this field still
keeps the human race within its traditional boundaries,
and Nature still holds the final citadel against all
comers. But now it is being undermined from all sides,
and changes its aspect almost from day to day, like an
erstwhile impregnable barrier that is crumbling away
before our eyes.
The years 1911-13 witnessed a convergence of
powerful new methods which, though their simultaneous
209
210 THE ULTIMATE STRUCTURE OF MATTER
development must be regarded as largely fortuitous, all
bear definite experimental testimony concerning the
hidden internal construction of the atom of matter. In
fact, we can now distinguish therein three distinct
regions, one within the other, between which probably
no interchange whatever of constituents occurs, but
through which, in succession, the atom makes the par-
ticular impression by which we recognise it in the
external world, and by which, in turn, it is successively
guarded from any direct influence from without. The
first, outermost region is that which the older sciences
of physics and chemistry have studied so minutely, and
which is directly concerned in endowing the atom with
most of those properties by which in the past it has been
recognised and studied. The second is an intermediate
region which can be reached and set into the vibration
known to us as X-rays, by the purely artificially generated
projectiles, the free-flying electrons or cathode-rays of
the Crookes tube, dealt with in Chapter IV. The last
and innermost region of the atom, or the nucleus, has
never yet been reached save by methods which we owe
solely to the study of natural radioactive changes and
by the projectiles, of such inimitable swiftness and
energy, which are spontaneously expelled during those
changes.
The Nature of Mass.
Actually before the coming of radioactivity, the dis-
covery of the electron, a particle more minute than the
smallest individual atom of matter, had given, in the
hands of Oliver Heaviside and Sir Joseph Thomson, a
possible clue to the nature of mass (p. 57). Without
any direct evidence that the mass of matter was, in
fact, due to this cause, the reasoning indicated that, if
the electron were sufficiently small — if the electric
charge of which it consists were concentrated into the
volume of a sphere of about 2 x 10"^^ cm. radius, which
is about one-hundred thousandth of the usually accepted
value for the radius of an atom — it would possess a mass
ELECTRO-MAGNETIC INERTIA 211
equal to that found — namely xTgo-th part of the mass of
the hydrogen atom, by virtue of thoroughly well-known
and understood electro-magnetic principles. A charge
of pure electricity, entirely unassociated with matter, as
the negative electron is believed to be, cannot be moved
from rest without an expenditure of energy, nor if
moving can it be brought to rest without yielding up its
energy. It, therefore, must possess inertia or mass. A
moving charge of electricity is indistinguishable from a
current of electricity. In the case of an ordinary electric
current " self-induction " opposes both its starting and
stopping. If we trace further the origin of the " self-
induction " in the case of a flow of the electric current,
or " electro-magnetic inertia " in the case of an indi-
vidual electron, both terms being technical expressions
for an identical action, we find it in a fundamental dis-
tinction between electrostatic and electrodynamic pheno-
mena— ^that is, between a charge at rest and one in
motion. The former has no magnetic properties, whereas
the latter has. The space surrounding a current of elec-
tricity, or a moving charge, is endowed with magnetic
properties, and the change in the surrounding space
when an electric charge, before at rest, is caused to
move, demands the expenditure of energy. This change
is believed to be transmitted outward from the electron
with the velocity of light. This endows a purely electric
charge with inertia or mass. So that a charge of pure
electricity must, if sufficiently small and concentrated,
simulate matter in its most fundamental attribute.
For the same charge concentrated into spheres of different
radius, the mass is inversely proportional to the radius.
Aie there, then, two kinds of inertia or mass, the one
" material " and the other " electro-magnetic," the one
for matter, still a fundamental, and the other for elec-
tricity, a derived conception that can be fully explained
by the phenomena known to attend its motion ?
212 THE ULTIMATE STRUCTURE OF MATTER
Sir Joseph Thojmson's Model Atom.
From this the idea arose naturally and was developed
by Sir Joseph Thomson, that atoms of matter might be
compounded of electrons in sufficient numbers to account
for their mass. For each atom nearly 2,000 electrons
per unit of atomic mass would be required. The prob-
lem of atomic constitution resolved itself into one of how
to maintain such systems of electrons in permanently
stable regime. The early attempts had little of reality
to recommend them, because by no known means could
such systems of electrons be held together without
assuming the existence of positive electricity in some
form. But positive electricity, existing like negative
electricity divorced from matter, refused to be dis-
covered, and, in fact, still remains unknown. In Sir
Joseph Thomson's model atom, the negative electrons
were supposed to revolve in orbits within a uniform
sphere of positive electrification of the same dimensions
as the atom. It had one very great and suggestive
merit, for it showed that the electrons would tend to
arrange themselves in rings. If the number of the
electrons were steadily increased, the newcomers would
incorporate themselves into the existing outer ring until
a certain number had been added, and then, if the
numbers were further increased, these existing rings
would become unstable, and the superfluous members
W^ould at a certain number suddenly rearrange them-
selves into a new permanent outer ring concentric with
those previously existing.
The Periodic Law.
This simulates very well the known facts with regard
to the elements as shown by the Periodic Table. Arrang-
ing the elements in increasing order of atomic mass we
get the well-known periodicity of chemical properties, the
tenth element resembling closely the second, the eleventh
the third, and so on, hydrogen being an exceptional
THE PERIODIC TABLE 213
element without analogues. So that all the elements
fall naturally into families, successive members in the
same family being separated from one another by seven
intervening elements in the early part of the table, and
by seventeen in the latter part of the table. The Periodic
Table is shown in Fig. 43. The elements are set down
successively in order of increasing atomic weight hori-
zontally, the vertical columns then contain families of
chemically allied elements. The separate places are
numbered consecutively at the top of the place. These
numbers are the so-called atomic numbers. Below the
name of each element is its chemical symbol and its
atomic weight. The families are numbered 0, I, II,
etc., and these " Group Numbers " express, with certain
reservations, the usual chemical valency of the element
— that is, the number of units of affinity with which it
enters into combination with other elements. Thus,
aluminium is in the IlIrd family and carbon is in the
IVth. When these combine it takes four atoms of
aluminium, each atom with three units of affinity, to
combine with three of carbon, each with four units of
affinity, the compound, aluminium carbide, being
represented by AI4C3. After the IVth group, the ele-
ments frequently combine to form compounds with
many different valencies. But here it may be stated,
though the simple rule is often not followed, that the
most usual valencies are either the group nimiber or
eight minus the group number. Thus, nitrogen either has
five valencies or three, chlorine one or seven, and so on.
That elements possess units of combining power, or
" bonds of affinity " as chemists call them, is one of
the numerous facts which has been, at least partially,
explained by the discovery of the electron and the fact
that electricity exists in atoms no less than matter.
Electrolytic Dissociation.
During the last quarter of the nineteenth century,
the theory of electrolytic dissociation, put forward by
28
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ELECTROLYTIC DISSOCIATION 215
Svante Arrhenius of Stockholm, became generally estab-
lished. It asserted that compomids of the class which
conduct the electric current in the state of solution, and
known as electrolytes, exist in solution in a more or
less completely dissociated condition, as oppositely
charged positive and negative ions, the migration of
which to the opposite poles constitutes the electric
current.
It would be idle to pretend that complete clearness of
interpretation has yet been attained, but the facts
appear somewhat as follow: The elements, sodium and
chlorine in Groups I and VII respectively, both act
usually as elements with a single unit of valency, but
they belong to, and are typical of, two distinct classes
of elements. Sodium is a typical basic, metallic, or
electropositive element, and chlorine is a typical acidic,
non-metallic, or electro-negative element. They com-
bine together with the utmost avidity to form common
salt, NaCl. But in solution in water we are forced, by
its behaviour to the electric current as an electrolyte, to
recognise that the complex NaCl does not exist as such,
at least for the most part. Rather, there are two new
particles, " sodion " and " chlorion," which exist apart,
and are called ions. The sodion — Na+ — is an atom of
sodium carrying one atomic charge of positive elec-
tricity, and the chlorion — CI" — is an atom of chlorine
with one atomic charge of negative electricity. It is as
though the act of chemical combination of metallic
sodium with the element chlorine was essentially the
transfer of an electron, or atom of negative electricity,
from the sodium atom, to the chlorine atom. The
sodium readily loses a constituent negative electron to
another element, such as chlorine, which will take it up.
But although equal numbers of positive and negative
ions may exist as separate particles when mixed together,
neither kind can exist alone. The enormous forces of
electrical repulsion between the similar charges, un-
neutralised by the presence of the opposite kind, effec-
tually prevent this being even conceivable. Whether
216 THE ULTIMATE STRUCTURE OF MATTER
an element loses or gains one or more electrons, however,
is not a self-contained property, but depends on the
nature of the other element or elements in the com-
pound formed, so that frequently elements in the
later families, V, VI, and VII, which may usually
tend to gain 3, 2, or 1 electrons and to act as acidic
elements, may act like basic elements and lose 5, 6, or
7 electrons.
Undoubtedly, in the broadest sense, though much is
not yet so clear, the chemical combining power of an
element is to be explained by the inherent tendency
the atom possesses either to attract from, or to yield
up to, another atom, one or more electrons. The act of
chemical combination, in some of the best-known and
typical cases, which, in an earlier day, was depicted as
due to the powerful attraction of one atom for and by
another, is primarily not exerted between the two atoms,
but between one of the atoms and the constituent
electron or electrons of the other. Between the two
material components of such a stable compound as
sodium chloride no cohesion or attraction probably
exists^
The molecule of sodium chloride, at least in the liquid
state, either when fused or dissolved, consists essentially
of two separate particles or ions, mixed together rather
than combined, which being oppositely electrically
charged, must always exist together in equal numbers
in order that the whole may remain electrically neutral.
But there is no definite bond of union other than this
purely electrical requirement, and this refers merely to
the aggregate number of each kind of particle rather
than to the individuals. Apart from this limitation,
the sodium and the chlorine in salt water exist separately
and totally uncombined for the most part. The in-
tensity of the electrical charge on an ion is almost incon-
ceivably greater than any known for matter in the mass.
It has been calculated that the mutual repulsion be-
tween the charges carried, for example, by the hydrogen
ions, would be sufficient to burst the strongest tube that
THE OUTERMOST ATOMIC REGION 217
can be made, long before there was forced in as much
hydrogen, in the form of ions, as would, in the ordinary
state, showthe hydrogen spectrum in a vacuum tube. This
assumes, of course, what is really quite impossible, that
such free ions could be put into a tube without being
discharged by contact with the walls of the tube.
The " chemical combination " of the partners in a com-
pound completely dissociated, as sodium chloride is in
liquid form, is due to a purely electrical and statistical
partnership of the otherwise completely independent
ions, which, in the modern view, is practically as effec-
tive in maintaining the combination as the rigid bonds
linking each individual sodium atom to one chlorine
atom which Dalton first pictured. This refers to the
class of electrolytically dissociated substances, which
comprises the acids, bases, and salts, and not to the very
large class of non- electrolytes, which comprises all the
organic compounds, where permanent individual unions
between the atoms of the molecule undoubtedly exist.
The Outermost Region of the Atom.
Chemical changes and chemical properties, in general,
deal only with the outermost region of the atomic
structure, and we shall not probably do violence to the
facts if, without at present attempting to review all
the evidence for this conclusion, we picture it as con-
taining a certain number of " valency " electrons.
This number is the same for all the members in the same
family or vertical row of the periodic table, and differs,
literally, unit by unit in passing hoiizontally from one
family to the next. For a certain number of electrons
in the outer ring — namely, that possessed by the zero
family — there is no tendency for the atom either to lose
or gain electrons. The members of this family, which
comprises the inert gases of the atmosphere, are totally
devoid of chemical affinity. The next family, in
Group I, which contains the alkali metals, has one
218 THE ULTIMATE STRUCTURE OF MATTER
electron more than this number, which is relatively
loosely held. In all probability it moves in an orbit far
external to all the rest. In the other direction, in
Group VII, containing the halogen family, the number is
one less than this number, and these elements readily
take up an additional electron in the presence of an
element of Group I which has such an electron in excess.
The outer ring of electrons seems for all atoms to try and
conform to a certain standard number. Atoms with
less rob the ones with more, and this process probably
constitutes, in the main, chemical combination. Whether
the robber and the robbed entirely part company, as in
the electrolytes, or remain interlocked, as in organic
compounds, is a secondary consideration. We may
suppose that, when the number of electrons in the outer
ring exceeds a certain limit, which in the first part of the
periodic table is seven, a complete new inner ring of
eight electrons is formed. The chemical properties,
however, depend only on the outermost ring directly,
and the inner rings exert a subordinate effect. The
valency of such an element and its general chemical
nature resembles, therefore, the eighth preceding
element. This holds in the early part of the periodic
table. At the 22nd element, titanium, a new and more
complicated dual periodicity commences, in which the
number of elements separating the consecutive members
of one family is eighteen instead of eight. A new group
of three closely allied elements, the so-called Vlllth
Group, now appears in the middle of the period, where
previously an argon element would appear, and the
next seven elements have a partial analogy to the seven
preceding the Vlllth Group. The easiest way of
regarding the matter is to suppose that ten metallic
elements, indicated in Fig. 43 between { } are inter-
polated into the old short periods.
At the 57th element, lanthanum, the law suddenly
and completely breaks down. A group of seventeen
elements, known as the rare-earth elements, and of which
two remain to be discovered, is interpolated into the
THE RARE-EARTH ELEMENTS 219
series at this point. They all resemble one another and
lanthanum with such extreme closeness that their separa-
tion and identification is one of the most laborious and
difficult tasks that the chemist can undertake. At
tantalum, the 73rd element, the series begins again
almost as if it had not been interrupted, and continues
normally to the end.
CHAPTER XIV
THE NUCLEAR ATOM
The Innermost Region of the Atom.
Now let us see what radioactivity can tell us of the
insides of these atoms, for be it remembered that though
the older chemical and physical properties of matter are
concerned only with the outermost shell, the seat of
government which impresses upon any atom its chemical
character, and which conditions that chlorine should
resemble bromine and differ from potassium, is inside
the atom, in a region impenetrable to the methods of
investigation known at the opening of the century.
From such methods we could only guess what might be
inside, and the guesses never even approached the truth.
But now we can send a messenger right through the
unknown territory, which perchance may, on re-
emergence, tell us something of more interest and value
to the race than any traveller who has ever struggled
back again into being from the waste places of the eartfi.
And this messenger, whose speed must be comparable
with that of light and whose mass must be comparable
with that of the atom it is to invade, is the a-particle
(Chapter IV.). We owe to the genius of Sir Ernest
Rutherford the recognition of the importance of this
new method of attacking the most fundamental of all
problems, that of the ultimate structure and constitution
of the atom. Together with his students, he has made
a close quantitative study of the effect on the os-particle
of its passage through the various atoms of matter.
Though, as Bragg showed, the a-particles pass straight
220
a-PARTICLES AS MESSENGERS 221
through the atoms, this is not the whole truth. Thou-
sands of a-particles pass through thousands of atoms in
their path, almost as if they were not there, suffering but
slight retardation and hardly any appreciable deviation
from their course at each encounter. But there occur
also, and as an exception, large deflections (compare
Fig. 19), and occasionally the a-particle is violently de-
flected through a large angle by an exceptionally close
encounter, like a comet passing round the sun. It may
even emerge from the side it entered. This is termed
" occasional large-angle scattering " to distinguish it
from the incessant very slight deviations, first in one
direction then in another, according to the laws of
probability, which, as a more minute examination has
shown, is continually happening to the a-particle as it
ploughs its way through the atoms. Inevitably this
makes us view the atom itself as consisting essentially
of a very small dense nucleus at the centre of a relatively
enormous and almost empty sphere of influence con-
taining only electrons. The a-particles, being immensely
more massive than the electrons, are not seriously dis-
turbed by the rings or shells of electrons whose revolu-
tions determine the apparent size of the atom as fixed
by the older physical methods. Against all other
invaders, these swiftly revolving satellites guard the
interior of the atoms as efficiently as if the atom really
occupies the space to the exclusion of everything else.
But such exclusive occupation of a definite volume of
space by matter is an illusion. A material projectile,
like the a-particle, moving at a speed the tenth of that
of light, passes through all the electronic ring-systems
as an errant sun might pass through the solar system.
This happens many thousands of times without any
serious consequences to the a-particles, or to the atomic
system invaded. But an occasional a-particle finds its
mark, and heads straight for the real atom — that is to
say, the central nucleus in which the material as dis-
tinguished from the electrical constituents of the atom
are concentrated.
16
222 THE NUCLEAR ATOM
The a-particle we know to be a helium atom of mass 4
carrying two atomic charges of positive electricity.
Or, more accurately, a helium atom is an a-particle
minus two electrons. In all probability the a-particle
is the simple nucleus of a helium atom, the central sun,
as it were, alone and unattended by any electronic
satellites or planets at all. The size of this central
nucleus of the atom, in relation to the apparent size
of the atom, is probably of the same order of magnitude
as that of the earth to the whole solar system. Ruther-
ford, for example, from these experiments, considers
that practically the whole mass of the hydrogen or
helium atom is contained in a central nucleus of dia-
meter one hundred thousand times smaller than the
accepted diameter of the atom. This central nucleus
carries a positive charge, to the extent of about one
unit, or atomic charge, of positive electricity, for every
two units of atomic mass. For each unit of positive
electricity resident in the nucleus a similar unit of
negative electricity, or an electron, revolves in one or
other of the outer shells, so that the negative charge on
the electronic systems is neutralised by the positive
charge on the nucleus or material core. This model of
Rutherford's differs essentially from the earlier models
in that it has been based on a careful and exhaustive
experimental examination of the single large-angle
scattering of a-particles.
Now let us consider the exceptionally close encounters,
when nucleus meets nucleus and large-angle deflection
of the a-particle results. If the atom invaded by the
a-particle is massive by comparison, the positively
charged nucleus constituting the a-particle will be
violently repelled as it impinges on the very much more
intensely positively charged and much more massive
nucleus of the heavy atom, and will be violently swung
out of its path, much as a comet is at perihelion. It is
true that the forces at work are repulsive rather than
attractive, but this makes no essential difference. If the
two nuclei happened to meet absolutely " head-on " the
H-PARTICLES 228
a-particle would be repelled the way it came almost at its
original velocity.
But when a-particles traverse atoms lighter than
themselves — for example, atoms of the gas hydrogen — a
different state of things must obtain. Here an abso-
lutely " head-on " collision would result in the hydrogen
atom being repelled in the same direction as that in
which the a-partlcle was travelling, but with a velocity
far in excess of that of the original «-particle. In fact,
this hydrogen atom will then behave as a new kind of
radiant particle, and by virtue of its smaller mass and
charge and greater velocity it should travel through the
hydrogen gas far further than the original «-particles
before being stopped. Marsden has shown that when
the a-particles are made to pass through hydrogen and
their range examined by means of a zinc sulphide
screen, in addition to the scintillations given by the
a-particles themselves, a few weaker scintillations, which
must be due to the repelled hydrogen atoms, can be
observed at distances from the source some four times
greater than the a-particles themselves are able to pene-
trate. These new particles may be termed " H-particles "
for the sake of clearness.
An Artificial Transmutation.
In 1919, by the work of Sir Ernest Rutherford, a
further important step in this advance was taken, which
raises the question whether a beginning has not already
been made in the achievement of actual artificial trans-
mutation to an infinitesimal extent. It has been recog-
nised, by the late Sir William Ramsay among others,
that, of all known agencies likely to be able to transmute
one element into another, the a-particle, on account of
its unique kinetic energy, was the most likely to prove
effective. The work described shows how exceedingly
difficult it is to hit the real atom exactly with the
a-particle. Later results have proved that only about
one out of 100,000 a-particles, in passing through one
224 THE NUCLEAR ATOM
centimetre of hydrogen gas at normal temperature and
pressure, produced an H-particle. Since, in this path,
the number of hydrogen atoms penetrated is 10,000, in
only one out of one thousand million collisions is the
nucleus of the atom of hydrogen really hit. In the rare
case when the a-particle actually impinges upon the
nucleus, it is to be anticipated that the latter, if not
an exceedingly stable system, might sometimes be
broken up.
Of the common gases, hydrogen, oxygen, carbon
dioxide, and nitrogen, which he exposed to the bom-
bardment of the a-particles, Rutherford observed an
anomaly in the case of nitrogen. These gases all gave
the expected effects, namely, the production of " N-
particles " and " 0-particles " — that is to say, new
rays were observed, longer in range than the a-rays,
which were first thought to be atoms of these ele-
ments with a single positive charge put into violent
motion by collisions of the a-particles with the nuclei
of the oxygen and nitrogen atoms, always in the
minute numbers to be expected from the results with
hydrogen. In these cases the range of the new particle
is only slightly longer than that of the a-particles them-
selves. But in nitrogen there were observed, in addi-
tion, particles of the long-range and other characteristics
exactly similar to the H-particles produced in hydrogen
gas. Only one such H-particle was observed for every
twelve N-particles produced. These results strongly
suggest, though they do not yet rigorously prove, that
the nucleus of the nitrogen atom struck by an a-particle
is occasionally shattered by the collision, and that
hydrogen atoms are produced from it. It may be sur-
mised, for example, as one possibility, that the nitrogen
atom of mass 14 is converted into a carbon atom of
mass 12 and two hydrogen atoms. The excessively small
proportion of the nitrogen atoms penetrated by the
a-particles, which are so shattered, must not be for-
gotten. This makes it exceedingly unlikely that such
a case of artificial transmutation, if it occurs, can ever
ARTIFICIAL TRANSMUTATION 225
be directly confirmed by direct chemical analysis. It
must also be remembered that in this case, even if it is
correctly interpreted, transmutation has not been really
artificially initiated. What has been done, at the most,
is to use a naturally occurring transmutation, that can
still be neither initiated artificially nor controlled, to
produce a secondary transmutation. The real problem
of how artificially to transmute one element into an-
other at will remains still completely unsolved.
While this book was passing through the press,
Ruth erf Old has published further results, in which
the real nature of these particles, generated by the im-
pact of the a-rays in different gases, has been examined
by the method by which the nature of the electron and
the a- and ^-particles has been established — that is
to say, the particles were subjected to the action of
electric and magnetic deviating fields, and, from the
magnitude of the deflection, the mass, the charge, and
the velocity were determined. This established the
correctness of the earlier conclusion that the H-particles
generated in hydrogen, and also in nitrogen, consisted
of singly positively charged hydrogen atoms. But it
was found that what have been termed " N-particles "
and " 0-particles " were not singly charged atoms of
nitrogen and oxygen, as first surmised, but for each the
same and an entirely new particle of mass 3, carrying
two positive charges. On the views discussed in the next
chapter they would appear to be atoms of an isotopic
variety of helium, otherwise unknown.
Thus the new results confirm the conclusion that the
nitrogen atom is shattered during a close nuclear
collision with an «-particle, but it appears to suffer
disruption in two independent ways, giving, in one way,
atoms of hydrogen of mass 1, and, in the other, atoms
of a new kind, of mass 8. In the case of the oxygen
atom the latter particles alone appear to be produced
(Sir Ernest Rutherford, Bakerian Lecture, Royal
Society, June 3, 1920).
226 THE NUCLEAR ATOM
Atoms compared and contrasted with Solar
Systems.
Thus, inevitably as science proceeds, the solid tangible
material universe dissolves before its touch into finer
and still finer particles, the unit quantities or " atoms "
of positive and negative electricity. The passive attri-
butes of matter in occupying a definite volume of space
to the exclusion of other matter resolves itself into an
active dynamic occupation by virtue of the sweep of
the electronic satellites in their orbits round the positive
central sun. But whereas, in the solar system in which
we live, the central sun is both large and massive in
relation to the sizes and masses of its attendant planets,
in the atomic solar systems there is a curious inversion.
From the facts disclosed in reference to the passage of
a-particles through hydrogen, it would appear that
the centres of the two colliding nuclei, the hydrogen
nucleus and the helium nucleus, approach to within
a distance of less than the accepted diameter of the
negative electron. The central material nucleus, in
which all but a negligible part of the mass of the atom
is concentrated, thus appears to be at least as small as,
and probably smaller than, the negative electron, the
smallest particle previously known to science. Since
the smaller the volume in which a given electric charge
is concentrated the greater will be its mass, it may really
be that the positive electron is very much more concen-
trated and very much more massive than the electron,
and that the nucleus of the hydrogen atom, the sim-
plest of all atoms, is in reality the missing positive
electron. But this, at present, is merely a suggestion.
The positive charge is the same in amount as the nega-
tive charge of the electron. For its mass to be that of
the hydrogen atom, which is 1,830 times that of the elec-
tron, its radius must be 1,830 times less, or about 10~^^ cm.
CHAPTER XV
ISOTOPES
Elements which are Chemically Identical.
In another totally distinct direction, radioactivity has
been the means of throwing a flood of light on the nature
of matter and in particular on the periodic law of the
elements, which epitomises the existing chemical know-
ledge of matter. In the first chapter, the underlying
limitations which attend all knowledge were empha-
sised. Such an underlying limitation is revealed by the
sequence of radioactive changes. In Chapter X. (p. 154)
it was shown that many of the known radioelements
resemble others so completely in their chemical nature
that no separation can be effected once they have been
mixed, and in Chapter XII. we came upon numerous
further examples of the same resemblance among the
members of the thorium disintegration series. No
chemist could detect by chemical analysis the separate
existence of the two uraniums, uranium I and II, or of
thorium and radiothorium, or mesothorium and radium,
or of lead and radiolead, in a mixture containing any of
these pairs. Naturally the question was asked whether
any of the common elements, for which radioactive
methods of analysis are not available, are, as supposed,
really homogeneous elements, and whether any are
mixtures of different elements, with different atomic
weights, but with identical chemical properties, so
merely appearing to be homogeneous to chemical
analysis. Matter is, in all probability, far more com-
plex than chemical analysis alone is able to reveal,
because radioactivity has shown us the existence
227
^28 ISOTOPES
of elements identical in their chemical behaviour,
but, nevertheless, distinct in atomic weight and in
stability.
The Periodic Law and Radioactive Changes.
In 1911 the writer pointed out that the products of
«-ray changes have a certain definite relationship in
chemical character to their parents. The chemical pro-
perties of an a-ray product correspond with those of an
element in the periodic table with group number two
less than that of the parent. Thus, radium in Group II
expels an a-particle and changes into the emanation in
Group 0, ionium in Group IV changes by expulsion of an
«-particle into radium in Group II, and so on. It was
also noticed that the passage through the periodic table
of the element undergoing change was frequently alter-
nating, the products frequently reverting in chemical
nature to that of an earlier parent. So radiothorium
resembles thorium, thorium X mesothorium I, and so
on. This curious atavism has now been very simply
and fully explained, largely owing to the chemical in-
vestigations of Alexander Fleck in the writer's labora-
tory at Glasgow, who spent three years in the exhaustive
study of the chemical nature of all the radioactive
elements, which survive for a long enough period for
their chemical nature to be determined, and many of
which had previously been very imperfectly investigated
from this standpoint. In consequence, the generalisa-
tion already alluded to in preceding chapters has come
to light. It was seen that the expulsion of a y8-particle
was entirely analogous to that of the expulsion of the
«-particle, but that, instead of the product possessing a
chemical nature corresponding with an element in the
periodic table with group number two less than the
parent, it corresponded with an element of group
number one greater. Hence if, in any order, one a-
and two /3-rays are expelled, the product is chemically
of the same nature as its parent, and the curious atavism
THE a- AND )8-CHANGE GENERALISATION 229
referred to above is explained. Radioactive children
frequently resemble their great-grandparents with such
complete fidelity that no known means of separating
them by chemical analysis exists. But, of course, the
two intermediate parents are readily separated. By
this means all the members of the family may be
recognised severally, although, but for this means, that
would be still impossible.
The complete generalisation, which was put forward
in 1913 independently during the same month by A. S.
Russell, K. Fajans, and the writer, is illustrated by
Fig. 44. The last twelve places of the periodic table,
from uranium to thallium, are placed consecutively side
by side, and the passage of the elements, in the uranium,
thorium, and actinium series, from place to place, as the
a- and /S-ray changes succeed one another, is indicated
by arrows. The figure is to be read at 45°, so that the
lines showing the atomic weights are horizontal.
Every detail of the chemical nature of the members
of the known sequences in the uranium, thorium, and
actinium series, including the complicated branchings
which occur towards the ends, bears out implicitly
these two simple rules. Independently of their origin,
atomic weights, and radioactive character — that is, of
the kinds of change they are about to undergo — all the
members of the three disintegration series, which, by
the consistent application of these rules fall into the
same place in the periodic table, are chemically com-
pletely identical and non- separable from one another.
Hence I have termed them isotopes or isotopic elements.
The Atomic Numbee.
Confining attention to the most generally important
consequences of this embracing generalisation, we may
at once connect the rules with the fact that the a-particle
carries a double positive atomic charge and the /3-
particle a single negative atomic charge. Each of the
successive places in the periodic table thus corresponds
280
ISOTOPES
Sequence of Changes of Uranium (U) and Thorium (Th) into various
Isotopes of Lead (Pb).
Fig. 44.
THE ATOMIC NUMBER 231
with unit difference of charge in the constitution of the
atom. This suggestion was made tentatively by van
der Broeck before it was first proved by these researches.
The discovery of the atomic nucleus by Rutherford
enables us to go further. It is hardly possible to doubt
that both the a- and the /S-particles are expelled from
the nucleus. Hence this difference of charge in the
constitution of the atom in passing from one place in
the periodic table to the next must be a unit difference
in the net positive charge of the nucleus of the atom,
and a corresponding unit difference in the number of
negative electrons external to the nucleus, which com-
pensates the positive nuclear charge and renders the
whole atom neutral.
In his original theory Rutherford concluded that the
magnitude of the positive charge of the nucleus was
approximately one-half of the number representing the
atomic weight of the elements. Now, from evidence
still to be considered, it is known exactly to be equal
to the number of the element in order of sequence in
the periodic table, when the elements are arranged in
order of atomic weight. This number is now always
called " the atomic number." Usually it is rather less
than one-half the atomic weight. Uranium, the last
element, occupies the 92nd place in the periodic table;
its atomic number is therefore 92, and its atomic weight
is 238.
So far as is known the atomic number of hydrogen is
one, that of helium is two, of lithium three, and so on
until we arrive at uranium, ninety-two. Gold is the
79th element, mercury the 80th, thallium the 81st, lead
the 82nd, and thenceforward, as shown in Figs. 43 and
44, by the numbers at the head of each place of the
periodic table.
IsoTOPic Elements.
The generalisation proves definitely that, as regards
the last twelve places in the periodic table, between
uranium and thallium, the successive places correspond
232 ISOTOPES
with unit difference of nuclear charge and unit differ-
ence in the number of external electrons as was pre-
viously assumed. But it also shows that in the ten
occupied places each place accommodates on the average
no less than four distinct elements. The atomic masses
of the various elements occupying the same place vary
in some cases by as much as eight units, and there is
nothing to show that the same may not occur through-
out the whole periodic table. Such groups of isotopic
elements, occupying the same place, possessing the same
net nuclear positive charge and the same number of
electrons in their external systems, are not merely
chemically identical and indistinguishable. Many of
their commoner purely physical characteristics, such as
spectrum and volatility, have also been found to be
identical.
The existence of such isotopic elements would not
have been suspected except for radioactive changes.
What fixes the chemical and general material character
of an element is a particular numerical charge, and this
charge is not the total charge of the atom, not even the
total charge of the nucleus of the atom, but is the net
charge of the nucleus or the difference between the
numbers of positive and negative charges which it con-
tains. The same net charge may be, and, in the case of
isotopes, is made up of different absolute numbers of posi-
tive and negative charges differing by the same amount.
When an a- and two y8-particles are successively expelled
the net charge becomes again what it first was, and the
position in the periodic table and whole chemical
character also reverts to the initial state. But the atomic
mass is different by four units, the mass of the a-particle
expelled.
The Problem of the Ancient Alchemist.
There is one interesting point that may be referred
to, which serves to show how nearly science has ap-
proached to the ancient alchemical problem of turning
THE PROBLEM OF THE ALCHEMIST 233
base metals into gold. In these spontaneous changes,
if either actinium D or thorium D had elected to expel
an a- instead of a /3-par»ticle, the product would have
been an isotope of gold instead of lead.
Gold occupies a position in the periodic table two
places removed from and before thallium, so that if
thallium could be induced to part with an «-particle,
the product would be an isotope of gold. If it was
sufficiently stable it would be gold for all practical
purposes. It is true its atomic weight and density-
would be somewhat greater, but otherwise it would be
the same. Or, again, if bismuth could be made to expel
two a-particles, or lead an a- and a /3-particle, gold again
would be the product. This, then, is a list of recipes
for the modern alchemist, one and all indubitable, but
one and all awaiting ^ means of accomplishment. It
remains for the future to show how the nucleus of an
atom can be so influenced as to be caused to eject an
a- or /S-particle at will. But it is a tremendous step
gained to know for the first time in what transmutation
really consists.
CHAPTER XVI
THE X-RAYS AND CONCLUDING EVIDENCE
The X-Ray Spectra of the Elements.
We now have to turn to yet another great advance.
Beginning with the case of ordinary light, it is well
known that it may be analysed into its component wave-
lengths by the use of a " diffraction grating," as well as
by an ordinary prism.
In the Rowland diffraction grating some large known
number, usually from ten to twenty thousand lines per
inch, are accurately ruled by a diamond mounted on a
dividing engine, upon a plane or concave surface of glass
in such a manner that all the lines are exactly parallel
and all precisely equally spaced apart. The light trans-
mitted by such a grating is split up into a large number
of parallel beams which " interfere " with one another,
and the result is that the direct beam is more or less
extinguished, but each different wave-length of light in
the beam is bent, or diffracted, from its course through
a definite angle which is different for each different
wave-length. So the light is resolved, or spread out,
into a pure spectrum much as when it passes through a
prism. Now, if the distance between the rulings — one-ten-
thousandth of an inch, for example — is exactly known,
the actual wave-length of each line in the spectrum may
be easily and exactly calculated. A beam of X-rays,
as we now definitely know, consists of a radiation of
precisely the same kind as light, but of wave-length
some ten thousand times shorter. Hence, to resolve it,
we would require the use of a " grating " at least a
thousand or ten thousand times more finely ruled thin
234
X-RAYS AND CRYSTALS 235
can be ruled by the most perfect dividing engine. Who
could make such a grating ?
But an infinitely more perfectly executed, and ten
thousand times more closely packed, assemblage than
the finest and most perfect Rowland grating ever made
was found in 1912, by Laue, Friedrich, and Knipping,
who discovered that the X-rays are regularly diffracted,
like light is by the grating, when reflected from the
surface of an ordinary crystal, such as rock salt, fluor
spar, calcite, and the like.
In this country the discovery was eagerly taken up,
and we owe to the Professors Bragg, father and son, a
clear insight into the whole subject. In the crystal, as
the crystallographers have, with eyes of faith, long
lepicted, the atoms of the substance are marshalled in
a definite space-lattice' of regular geometric form, so
that each atom is fixed at a definite point in space at a
definite distance from and in a definite angular direction
to all the atoms surrounding it. The smallest number
of atoms required completely to represent the pattern
— so that the whole structure is made up simply by
redupHcating this unit indefinitely in the three dimen-
sions— is called the space-lattice of the crystal. More-
over, the distances between the atoms, or points, of the
space-lattice is, for common crystals, just of the right
order of length to resolve the X-rays in a manner pre-
cisely analogous to that in which light is resolved by the
Rowland diffraction grating. If we know for any one
crystal what this distance actually is, we can determine
the wave-length of any X-ray from the angle at which
it is reflected from the crystal.
For the ordinary heterogeneous beam of X-rays
given by an ordinary X-ray tube, which corresponds
to white light, the beam is resolved by the crystal
into an X-ray spectrum, and the wave-length of
the component radiations may be found. If we
know the wave-length for any one X-ray, we can
find out for any crystal, in any plane or face we
choose, the precise distance apart between the atoms
236 THE X-RAYS
that make it up, and so we can construct its space-
lattice.
This has given crystallographers a powerful direct
method of testing the reality of the space-lattices which
have been arrived at by theoretical reasoning and the
power of second sight of the mathematical mind. The
results already have been gratifying and remarkable.
The actual spacial arrangements of the individual atoms
that go to make up the crystal are now being precisely
measured and explored, and, as has so frequently hap-
pened before, the patient theoretical conceptions of a
generation less brilliantly equipped with experimental
methods of inquiry are being triumphantly vindicated.
But it is not with this field we are now most closely
concerned. It is rather with the wave-length of the
X-rays, and with their period or frequency, which can
be so found by this method. If we consider the unit
of time, one second, in this period light or X-rays travel
Sxio^*^ cms., whatever the wave-length. But in the
3 X 10^° cms. of length, or second of time, there will be
about twice as many separate waves of violet light as of
red light, and many thousand times more waves of any
X-ray than of either. From the wave-length we can at
once find the frequency or " pitch " of the radiation,
or the number of vibrations per second to which it
corresponds. This frequency again, in atomic solar
systems, corresponds with the number of revolutions
made per second by the electron in its orbit within the
atom, and this depends on the diameter of its orbit.
In much the same way we might speak of the frequency
of a planet as the number of revolutions it makes round
the sun in a century, and this depends on the distance of
the planet from the sun. The rays that constitute the
ordinary visible spectrum arise probably from the
outermost electrons of the atom, the ones, that is, that
are responsible for the chemical character and which
traverse orbits of diameter of the order of 10"^ cm.,
which is the diameter of the atom, meaning by that the
whole atomic system. To get waves of a thousand to
RESOLUTION OF y-RAYS 237
ten thousand times shorter length, and frequencies a
thousand to ten thousand times greater than for visible
light — to get X-rays, in fact — it is clear that we have
to get much nearer to the centre of the atom, into a
region intermediate between that in which the ordinary
phenomena of physics and chemistry originate and the
innermost nucleus disclosed by radioactivity.
The 7-Rays.
By the same method of reflection from crystal sur-
faces some, at least, of the 7-rays have also been resolved
and shown to be X-rays, but of very much shorter wave-
length in general than those artificially produced. The
wave-length of light is usually expressed in Angstrom
units (written A). One Angstrom unit is equal to
10"^ cm. The wave-lengths of visible light waves vary
from 6,000 or 8,000 (A) in the red to 3,500 (A) in the
violet, and to 2,000 in the extreme ultraviolet. ^The
wave-length of the X-rays range from, perhaps, 8 A for
very soft X-rays to 0-5 A for the most penetrating type
that can be produced. But the wave-length of 7-rays
is in general much less ranging from 1-2 A to as little as
0-07. Moreover, it is believed that for the most typical
very penetrating 7-rays of radium and thorium the
wave-length is far too short even for the crystal to be
capable of resolving them, and they may have wave-
lengths 100 times shorter than the shortest yet resolved.
The existence of rays so short in wave-length and high
in frequency points to a revolution of electrons in the
atom in orbits of excessively minute diameter, so
minute that the question arises whether the 7-rays do
not really originate from electrons actually contained
within the atomic nucleus. These results furnish
another and independent proof that radioactive pheno-
mena occur entirely in the atomic nucleus.
17
238 THE X-RAYS
The Intermediate Region of the Atomic Struc-
ture. — The Homogeneous Characteristic
X-Rays of Barkla.
A Rontgen tube gives X-rays of all wave-lengths
within limits which depend on a variety of conditions,
such as the nature of the metal constituting the anti-
cathode, the degree of vacuum, and the potential differ-
ence between the electrodes. The very important dis-
covery was made by Barkla that, when such X-rays
impinge upon various metals, they will, if penetrating
enough, produce new secondary homogeneous X-radia-
tion, the properties of which are characteristic of the
metal and not of the primary radiation. Each element,
except those of less atomic weight than sodium, emits
under such circumstances an X-ray of definite and
characteristic spectrum, which differs from the ordinary
light spectrum given by the same element in being ex-
cessively simple. Often it consists of a single strong
line together with one or more weaker ones. Such
characteristic X-rays belong to various series, designated
the K-, L-, M-series connected in the following way:
Beginning with sodium, the 11th element in the Periodic
Table, the X-ray, characteristic of the element sodium,
belongs to the so-called K-series, and is extremely
feebly penetrating and of long wave-length, as the wave-
lengths of X-rays go. Going up through the elements
in increasing order of atomic weight, as far as tin, the
50th element, the K-radiation produced steadily dimin-
ishes in wave-length and increases in penetrating power,
until, at tin, it is difficult artificially to generate a
primary X-ray of sufficient penetrating power to excite
the characteristic radiation. Hence this experimental
limitation prevents this series being studied for elements
of greater atomic weight.
Before this, however, beginning with the element zinc,
the 30th element, in addition to the K-radiation, a new
characteristic radiation of very feeble penetrating power.
BARKLA'S X-RAYS 239
belonging to the so-called L-series, makes its appearance.
From zinc onward this new radiation increases in pene-
trating power and decreases in wave-length until the
last element uranium is reached. Again, at gold, the
79th element, another new series, the M-series, is first
observed, very non-penetrating at first, but increasing
in penetrating power to uranium.
Moseley made a systematic determination of all the
wave-lengths of the principal lines of these characteristic
X-rays from aluminium to silver in the K-series, and
from zirconium to gold in the L-series, and discovered that
they are connected together by a simple mathematical
relation, involving the atomic number of the element.
The square-root of the frequency (as we have seen the
frequency is proportional to the reciprocal of the wave-
length) is proportional to a number that increases by
one in passing from any element in the periodic table to
the next. In other words, the square root of the fre-
quency is proportional to a number that increases in the
same way as what we have termed the atomic number
of the elements, when arranged in order according to
the Periodic law.
The practical value of this discovery was great. For
the first time it was possible to call the roll of the
chemical elements and to determine how many there
were and how many remained to be discovered. There
are between hydrogen and uranium ninety-two possible
elements, of which only six remain to be found —
namely, the two unknown heavier analogues of the
element manganese, two rare-earth elements, and the
two heaviest analogues of iodine and caesium re-
spectively (see Fig. 43).
It is curious that the first two should still and for so
long elude discovery. They would in all probability be
most useful metals, allied to the noble metals in char-
acter, the first to the light platinum metals, ruthenium,
rhodium, and palladium, and the second to the heavy
platinum metals, osmium, iridium, and platinum.
As is well known, the Periodic Table comprises certain
240 THE X-RAYS
exceptions. Tellurium has an atomic weight higher than
iodine, though in the periodic table it precedes it, and
the same is true for argon and potassium, and for cobalt
and nickel. The X-ray spectra of these elements con-
firms the order in which they have been put by chemists
in the periodic table on account of their chemical char-
acter and despite their atomic weights. This shows
that it is the atomic number — i.e., the net positive
nuclear charge of the element, or the number of elec-
trons external to the nucleus — which fixes the position in
the periodic table, rather than, as hitherto supposed,
the atomic weight. The existence of isotopic elements
of identical chemical character but different atomic
weight points to the same conclusion. In fact, this
work on X-ray spectra dovetails perfectly into the con-
clusions reached, independently, in the study of radio-
active change, and extends them to all the elements in
the periodic table.
The Atomic Mass or Weight.
The chemical character, and even the spectrum of an
element, at least to a degree of approximation attainable
by common methods, depends upon the atomic shell
and not upon the atomic nucleus, and the character
of the shell is identical, whatever the nucleus, so long
as the atomic number is the same. The atomic mass or
weight, on the other hand, on the views adopted, is to
all intents and purposes a property of the nucleus alone.
Mass and radioactivity, the oldest and the newest pro-
perties of matter, are in this respect allied and sharply
to be distinguished from all the other properties. Iso-
topes have in general nuclei of different mass but the
same net positive charge, and therefore their outer
electronic systems and all the properties which origin-
ate therein — that is to say, all properties save mass
and radioactivity — are practically identical and indis-
tinguishable.
We have seen that radioactive change afforded a very
ATOMIC WEIGHT OF LEAD 241
subtle way of separately distinguishing between and of
actually separating isotopes in favourable cases. In the
disintegration sequence A>B^C>D>, A, B, C are neces-
sarily elements completely distinct chemically and
capable of easy separation by chemical analysis. But
if in the three changes, one a- and two yS-particles are
expelled, D is necessarily chemically identical with A,
but of atomic mass four units less.
Because of the change, D can be apprehended as an
individual, and, since B and C are separable from A and
in course of time turn into D, in cases when the periods
are favourable, D can be separated from A. Except for
the change, A and D would, in spite of the difference in
their atomic weights, be mistaken by chemists, relying
on the usual chemical and spectroscopic criteria of purity
and homogeneity, for a single homogeneous element.
Its atomic weight would be a mean of the atomic weights
of its constituents, depending not only on the magni-
tude of each, but on the proportions in which they were
mixed. This would apply not merely to the radio-
elements but equally to all. It is therefore, perhaps,
not altogether surprising that all the many efforts made
to find exact numerical relations between the atomic
weights of the various elements should have proved
fruitless.
The Element Lead.
These ideas have been put sharply to experimental
test in the case of the element lead. As the generalisa-
tion illustrated by Fig. 44 shows at once, the ultimate
products of all the disintegration series in all branches,
so far as they have been traced, end in the same place in
the periodic table — namely, the place occupied by lead.
Therefore, in spite of the differences of origin and of
atomic weight, they must all be isotopes of lead, if the
apparent ends of the series coincide with the actual ends
and no further, as yet undetected, changes occur.
The atomic weight of ordinary lead is 207-2, whereas
that of the main branch of the uranium series, is 206,
242 THE X-RAYS
and that of both the branches of the thorium series is
208. The atomic weight of the end product of the
actinium branch series is doubtful, but, as it is only
present in small relative quantity, it may be, in the first
place, neglected. Clearly, if this view is correct, the
lead derived from a uranium mineral ought to have an
atomic weight somewhat lower than that of ordinary
lead, and the lead derived from a thorium mineral an
atomic weight somewhat higher. The prediction, like
so many that have been made in this subject, has been
completely confirmed by experiment. Lead from care-
fully selected uranium minerals, not containing thorium
in detectable quantity, has been found to have an atomic
weight as low as 206-05. Lead from carefully selected
thorium minerals containing only a small quantity of
uranium has been found to have an atomic weight as
high as 207-9. Chemically, they are identical and in-
distinguishable from common lead, which, indeed, may
well be a mixture of these two isotopes in the right
proportion to give an atomic weight of 207-2 !
Their spectra, for all practical purposes, are identical
with one another and with that of ordinary lead. But,
quite recently, a minute difference of wave-length has
been established in the case of one of the brightest lines,
a difference that does not exceed one part in ten million
or one-thousandth of the difference between the two
sodium lines Di and D2. It is so minute that it can
only with difficulty be established by the most refined
measurements. Nevertheless this difference between
the spectra of isotopes is likely to prove of great
importance.^
Agreeably, however, with what is to be expected for
isotopic atoms having identical shells but nuclei of
^ The ingenious suggestion has been made that it might be used to
separate the isotopes of chlorine (p. 248). A beam of Hght filtered
through chlorine will lose first the vibrations corresponding with those
of the lighter isotope, since it is in predominant quantity, and may
then be able to stimulate the heavier isotope only to react with hydrogen,
thus effecting the separation. This is being tried at Oxford, and at
the time of writing (July, 1920) the results appear most promising
(T. R Merton and H. B. Hartley, Nature, March 25, 1920).
THE ATOMIC WEIGHT OF IONIUM 243
different mass, the densities of the different kinds of
lead are different just in proportion to the differences in
atomic weight. In other words, the different isotopic
atoms have the same volume.
Another precisely parallel case has been established
for the isotopic elements ionium and thorium. We have
seen (p. 153) that, on account of its period being forty
times longer than that of radium, the amount of ionium
in a mineral must be something like 12-5 grams per ton
of uranium, or 58 grams per gram of radium. Now all
uranium minerals yet examined on a sufficiently large
scale contain, probably, a larger quantity of thorium
than this. It is a suggestive and unexplained point
that the proportion is smallest in the secondary recent
uranium minerals. In practically all the primary
uranium minerals several per cent, of thorium is found.
Thus, ionium can never be obtained pure free from its
isotope, thorium, but from a suitable secondary uranium
mineral, a preparation containing a considerable pro-
portion of ionium, admixed only with thorium, may be
separated. Such a preparation separated from 30 tons
of Joachimsthal pitchblende by Auer von Welsbach, has
been investigated by Honigschmid. For pure ionium
an atomic weight, 230, is to be expected, since it changes
into radium with expulsion of an a-particle. The atomic
weight of the ionium-thorium mixture described was
found to be 231-51, whereas that of pure thorium, by
the same method, was 232-12. But the spectrum of the
thorium-ionium preparation was, so far as could be
seen, identical with that of the pure thorium preparation,
and in both no impurities whatever could be detected.
The elimination of everything but ionium from the
thorium by the elaborate chemical purifications adopted
in the treatment of the material had been effected, but
these methods are incapable of affecting, to the slightest
degree, the ratio of the ionium to the thorium.
244 THE X-RAYS
Separation or Isotopes.
It would be idle to deny that these new ideas, that
different nuclei may exist in atoms which, to the chemist
and spectroscopist, are indistinguishable and insepar-
able, cuts far more deeply into the basis of chemical
theory than did the discovery of the actual disintegra-
tion of the radio-elements and of the spontaneous evolu-
tion of one element from another. It is of interest to
inquire into the possibilities of separating a mixture of
isotopes, or, if this is impracticable, of detecting their
separate existence in a mixture without separating
them. It will be obvious that any property which
involves directly the atomic mass could, theoretically,
if not practically, be employed for their separation and
separate detection. But it is remarkable how difficult
such methods are to apply to this purpose, and how few
of them ever have been used as practical aids to chemical
analysis. The rate of diffusion of a gas, or, less suitably,
of a substance dissolved in a liquid, depends directly on
the molecular weight of the substance and therefore of
the weight of the separate atoms the molecules contain.
Theoretically, thorium and ionium, the two uraniums or
common lead itself, if it is a mixture of isotopes as is
possible, ought to be capable of resolution by diffusion
methods. But this has not yet been practically achieved.
Other methods, such as depend upon centrifuging the
mixed material, or submitting it to the process of
thermal diffusion, have been proposed but not yet
successfully carried out.
It would be an extraordinarily difficult and laborious
piece of work, for example, to separate the constituents
of the air in a pure state by diffusion, though a partial
and incomplete separation by this means might easily
be effected. It is not a method a chemist would
employ unless he were obliged. On the other hand,
though on the point there is a difference of opinion, any
commonly used purely physical method other than those
NEON AND METANEON 245
mentioned, such as fractional distillation, crystallisation,
or adsorption, is not likely, even theoretically, to be
effective in separating a mixture of isotopes. These
certainly depend upon the chemical character of the
element, rather than its atomic mass.
Neon and Metaneon.
Interesting, because it was discovered just at the
time that the true interpretation of isotopes had been
found, and also because it concerns an element very far
removed from the heavy elements at the end of the
periodic table undergoing radioactive change, is the case
of neon and metaneon. The element neon is one of the
inert gases, similar to argon, existing in the atmosphere
to the extent of some twelve parts per million by volume.
It is intermediate, in the zero family of elements, between
helium with atomic weight 3-99 and argon with atomic
weight 39-9, exactly ten times greater — both these being
practically whole numbers. The atomic weight of neon
is 20-2, a number differing from the nearest integer by
a fifth of a unit.
As a sequel to his classical work (p. 57) in elucidating
the charge and mass of the electron, which constitutes
the cathode ray of the vacuum tube, Sir Joseph Thomson
applied similar methods to the positively charged par-
ticles, or " positive rays " as they are called, which under
certain circumstances can also be detected in the
vacuum tube discharge. Here in every case so far
examined the mass of the particle is never less than
that of the hydrogen atom, and often it is much greater.
In fact, so was developed a novel method of determining
the atomic mass of elements, such as hydrogen, oxygen,
nitrogen, and other gases which are present as positive
ions in the vacuum tube discharge, and the molecular
weight of such particles as so exist in groups of more
than one atom. One of the most interesting of the
numerous discoveries made was that of the gas called
X3, which has a mass three times that of the hydrogen
246 THE X-RAYS
atom, and which is, in all probability, the molecule H3,
analogous to ozone, the allotropic form of oxygen, O3,
though chemists have never yet prepared or observed
the existence of such an allotrope of hydrogen. ""■ But the
same is true of many groups, such as CH, CH2, CH3, for
which this new and exceedingly delicate method of gas
analysis indicates at least a passing existence.
The interest of this method, depending as it does
directly upon the mass of the atom or molecule, from
the present point of view is that, undoubtedly, it would
be capable of revealing, if they existed, in any gaseous
element, the separate individual components of a mix-
ture of isotopes of different atomic mass. It is, in fact,
almost the only practical method that could do so
without ambiguity. Now, in examining the positive
rays produced in neon by the electric discharge. Sir
Joseph Thomson and Mr. Aston found in addition to the
neon atom carrying a single positive charge, Ne+, of
mass 20, a much fainter indication of another atom
with a single + charge, of mass 22, which provisionally,
as it could not be ascribed to a known element, they
attributed to a new gas which they named metaneon.
The question at once arose whether this was a case of
the isotopism with which we have become familiar in
the case of lead and the radio-elements. An attempt
to separate neon and metaneon from ordinary neon, by
a prolonged series of fractional absorptions of the gas
in cooled charcoal, effected no separation whatever.
The density of the fractions separated by the process
were identical and the same as before the treatment,
whereas metaneon, with atomic weight 22, should have
a density 10 per cent, greater than neon with atomic
mass 20. But this, as we have seen, is to be expected
of isotopes, for in all probability the ordinary physical
properties, such as volatility, etc., are, hke the chemical
i This differs from the new particle of mass three more recently
obtained by Rutherford in the bombardment of oxygen and nitrogen
atoms by a-particies, in that it carries a single instead of a double
positive charge.
ISOTOPES GALORE 247
properties, indistinguishable. Neon remains still un-
resolved into its two components, though after a long
series of fractional diffusion experiments some in-
dication of a partial separation was obtained.
But the latest information confirms the existence of
metaneon in the gas. Aston has developed the positive
ray method of analysis considerably, so that it is capable
of fixing with great precision the atomic or molecular
weight of the particle causing the positive ray. His
measurements showed neon to be a mixture of two gases
of atomic weight 20-00 and 22-00 to within an error of
one part in a thousand. So we may conclude with
considerable probability that these two isotopic gases,
in proportion of about 90 per cent, of the first and
10 per cent, of the second, constitute the ordinary
element neon derived- from the atmosphere.
The General Prevalence of Isotopism.
At the time of correcting the proofs of this book
(July, 1920), this work of Aston has developed into one
of the most important contributions of recent times to
our knowledge of the chemical elements. The new
methods, a brilliant outcome of combined mathematical
and experimental ability, have proved themselves to be
of extraordinary power and accuracy in the detection of
isotopes and the measurement of their separate atomic
weights. By altering the mode of application of the
electric and magnetic deviating fields, an effect of the
utmost practical service, analogous to the focussing effect
of an ordinary lens on light, was secured, whereby all
the particles of the same mass and charge in a narrow
diverging cone of positive-rays are brought to a focus
at a point, the foci for different particles lying on a
straight line, in the plane of which the photograhhic
plate is put. Each particle thus records its position
as a spot or line on the plate, and there results an analysis
of the beam into its different constituent particles, quite
analogous to the resolution of light into constituent
248
THE X-RAYS
lines in a spectrum. From the position of the lines on
the photographic plate, the mass of the atom producing
it can be determined with an accuracy scarcely, if at all,
inferior to that attained by chemical methods in the
finest atomic weight determinations. But the method
has the added inestimable advantage that mixtures of
isotopes show their several atomic weights rather than
the mean value, which is all that can be got from
chemical determinations.
The results of this new method so far announced are
sufficiently startling. Eighteen elements have, as yet,
been examined. Of these, nine only were found to be
homogeneous. The other nine consist of mixtures of
from two to as many as five or more isotopes. More-
over, in every case, except hydrogen, the true atomic
weight is found to be an exact integer (in terms of the
atomic weight of oxygen as 16, taken as the standard
of comparison) to an accuracy of one part in a thousand.
For hydrogen, the atomic weight on this basis, 1-008,
deduced by chemists from some of the finest atomic
weight work ever performed, has been exactly con-
firmed. The results are collected in the table below.
" Pure "
Atomic
'■' Mixed ^^
Number of
Atomic
Elements.
Weight.
Elements.
Isotopes.
Weights.
Hydrogen
1-008
Boron
Two
10-00 and 11-00
Helium
4-00
Neon
Two ,
20-00 and 22-00
Carbon
12-00
Silicon
Two or three
28-0, 29-0, and
(?)30-0
Nitrogen
14-00
Argon
Two
36-0 and 40-0
Oxygen
16.00
Chlorine
Two
35-0 and 37-0
Fluorine
19-00
Bromine
Two
79-0 and 81-0
Phosphorus . .
31-0
Krypton
Five or six
78 (?), 80, 82, 83,
84, and 86
Sulphur
320
Xenon
Five (?)
128,130,131,133,
and 135
Arsenic
750
Mercury
Five or more
202, 204, and
three or four
unresolved be-
tween 197 and
200
As shown by the intensities of the different lines, the
proportion in which the isotopes are present accord
PROBLEM OF TRANSMUTATION 249
well in each case with the value of the mean atomic
weight as determined chemically. Thus the two isotopes
of bromine are in similar proportion, but the lighter
isotope of argon is barely detectable. It is thus not
too much to suppose that all the atomic weights, except
hydrogen, are exact integers, and that the fractional
values found by chemists for some of the elements are
due to their being mixtures of several isotopes.
The Problem of Transmutation.
From the picture we have formed of the general
structure of the atom and the view we have of what
exactly would constitute a transmutation, we may
attempt, in conclusion, to consider the kind of methods
by which its accomplishment might practically be
attempted. It is clear that it is the nucleus of the atom
that has to be changed, either by adding to or sub-
tracting from it positive or negative charges. The sub-
traction or addition of electrons, so far as the outermost
shell of the atom is concerned, in no sense constitutes a
transmutation, but is what occurs in ordinary chemical
changes. In the free state of the element the atom is
electrically neutral. The number of external electrons
is equal to the net positive charge of the nucleus.
Subtraction of one " valency " electron or more from
the outermost shell produces the positive ion, which is
characteristic, not of the free element, but of it when
combined with other elements to form chemical com-
pounds. But such additions and subtractions are con-
fined to the outermost shell. There is no exchange yet
capable of being effected between the electrons in the
inner completed rings and either the electrons in the
outermost ring or the electrons inside the nucleus.
When, however, the nucleus spontaneously ejects posi-
tive or negative charges, as it does in the a- and /3-ray
changes, a complete and instantaneous rearrangement of
the electrons both in the completed rings and the outer
shell appears to follow. In brief, to transmute an atom,
250 THE X-RAYS
the change has to be effected from within, outwards
from the central nucleus. It cannot, at least as yet,
be impressed upon the nucleus by any changes in the
exterior electronic shell, imposed from without.
But the comparative ease with which the outer shell
of the atom may be altered by chemical and also by
electrical forces imposes in itself a formidable practical
barrier to any more deep-seated change.
We have seen that the a-particle may be regarded as
the agent most likely to break up the nucleus of an
atom if it impinges upon it, and that this actually may
occur in the case of the nucleus of the nitrogen atom.
Is it possible artificially to generate an a-particle or one
possessing a similar amount of kinetic energy ?
It may be calculated that the energy of the a-particle,
over the range of velocity so far studied, is such as it
would acquire in passing between two points differing
in electric potential by from two to four million volts.
This gives a quantitative idea of the strength of the
electric field likely to be required before particles anal-
ogous to the a-particle could be successfully produced.
We may be fairly certain that the only influences
likely to be effective in transmuting matter will be
electrical in character, and that very much higher poten-
tials at present known or utilised in electrical engineering
will have to be developed before there is much chance of
success. Along this road much that is new and impor-
tant will first have to be made clear. So far as it has
been followed, a barrier to further progress has been
reached, which may or may not prove to be fundamental.
The attainment of very high potentials at present seems
to be limited by the failure of the insulation. Even a
practically perfect vacuum, it appears, fails to insulate,
and transmits a discharge across it when the potential
exceeds a certain limit.
Moseley hit upon the very ingenious idea of using the
radium clock (Fig. 15, p. 59), as a method of arriving
simply at otherwise unattainable potentials. If the
clock there depicted is deprived of its leaves, if the
CONCLUSION 251
insulating support of the radium can be made good
enough and the vacuum sufficiently nearly perfect, there
ought, theoretically, to be no limit to the extent the
radium would become positively charged, and therefore
to the difference of potential between it and the sur-
rounding wall, unless, thereby, the radium products
were prevented from further disintegrating and emitting
their /3-rays.
In practice Moseley could not, with his particular
apparatus, attain a potential much above 150,000 volts.
A discharge through the vacuum always occurred at this
point.
The reason probably is that the loosely held " va-
lency " electrons in the outermost shell of the atoms
constituting the surfaces are dragged out of the atom
by the electric field so causing the discharge. Such a
change is not transmutational, but is allied to or identical
with that produced by ordinary chemical agencies. It
indicates that there is a definite limit to the extent to
which matter can be charged, and at present this rather
closes the door to further progress.
The outer regions of the atom effectively guard the
inner from being attacked. If a perfect vacuum is
unable to withstand the electric forces without trans-
mitting the discharge, it may be expected that any
material insulator is even less likely to do so.
Conclusion.
This must conclude the attempt to deal with the
numerous and important advances made since these
lectures were first given. The field of work has opened
out in a number of directions previously unsuspected.
The problem of transmutation and the liberation of
atomic energy to carry on the labour of the world is no
longer surro.unded with mystery and ignorance, but is
daily being reduced to a form capable of exact quanti-
tative reasoning. It may be that it will remain for ever
unsolved. But we are advancing along the only road
252 THE X-RAYS
likely to bring success at a rate which makes it probable
that one day will see its achievement.
Should that day ever arrive, let no one be blind to the
magnitude of the issues at stake, or suppose that such an
acquisition to the physical resources of humanity can
safely be entrusted to those who in the past have con-
verted the blessings already conferred by science into a
curse. A.S suddenly and unexpectedly as the discovery
of radioactivity itself, at any moment some fortunate
one among the little group of researchers engrossed
in these inquiries might find the clue and follow it
up. So would be diverted into the channels of human
consciousness and purpose the full primary fountain of
natural energy at its source, for use or misuse by men,
according as to whether the long and bitter lessons of
the painful past and present have even yet been really
learned.
INDEX
A
a-particles, Collision of, with
matter, 62-67, 223, 224
— Bombardment of gases with,
224
— Coloration of mica and gems by,
165
— Connection of, with helium, 44,
60, 93-104,
— Energy of, 61
— from radium itself, 94
— from the emanation, 79, 144
— from uranimn, 149
— Individual, 42, 44-46, 61
— Limiting velocity of, 61, 66
— Mass of, 60, 98
— Niunber of, expelled by radium,
40, 42, 45
— passage through atoms, 220-
223
— Positive charges carried by,
60, 63
— Proof of identity with helium
of, 102
— Scattering of, 63, 222
— Tracks left by, 65
— Velocity of, 61, 66, 94, 161,
221, 249
a-ray product, chemical properties
of, 228
a-rays, 41-67
— Absorption of, 33
by air, 34
— Connection between range of,
and period of substance, 164
— Magnetic deflection of, 60
— Making paths of visible, 64
— Range of, 34, 45, 133, 161, 164
in mica, 166
— Resolution of, 41-46
Accumulation of products, 95-98,
123
Actinivun, disintegration series,
186, 198-204, 207, 214. 229
— Emanation, 198, 203
— Origin of, 199, 204
— Parent of, 205
Actinium, period of life of, 198
— Production of helium from,
99, 102
— A, 198, 203
— B, C, and D, 198
— X, 198
Active deposit of actinium, 198-
204
of thorium, 194-197
of radium, 137-144
Residual activity from,
145
Age of the earth, 25, 75, 98,
177-183
Ages, The geological and incan-
descent, 179
Alchemist, The problem of the,
232
Alkaline-earth elements, 85, 105
Alternative theories of radio-
active energy, 68, 89
Aluminium, 205, 213, 214
— carbide, 213
Analogies between the disinte-
gration series, 188-191, 198
Angstrom units, 237
Antimony, 214
Anionoff,G.N., 205
Argon, '84, 85, 97, 105, 214, 240
— atomic weight, 248
Arrhenius, Svanie, 215
Arsenic, 214
— Atomic weight of, 248
Aston, F. W., 246, 247
Atom, Definition of, 105-108
— Innermost region of, 220
— Intermediate region of, 238
— Model, 212
— Nuclear, 220
— Outermost region of, 217
— Structure of, 210
Atomic disintegration, 39, 58, 67,
89, 94, 96, 98, 105, 109,
112, 155, 157, 168, 209
Cause of, 14
Multiple, 200
— mass or weight, 240, 248
253
18
254
INDEX
Atomic number, 231, 239, 240
— property. Radioactivity an, 12,
13, 15, 68, 74, 83, 110
— synthesis, 180, 208
Atoms, 2, 12, 40, 46, 60, 63, 66,
84, 105, 158-161
— Interpenetration of, 63
— Passage through, of a-particles,
220-223
Atoms, Solar systems compared
and contrasted with, 226
Autunite. 98, 148
Average life, Determination of,
115
of common elements, 157
of emanation, 113
of ionium, 134
of radium, 117, 125, 207
of thorium, 207
of uranium, 116, 125, 207
Period of, 112, 207
B
/3-particles, 49-59
— Charge of, 50, 51, 58
— Mass of, 57
— Tracks left by, 65
— Velocity of, 58
)3-ray product, Chemical proper-
ties of, 228
j8-rays, 29-66, 228, 250
— Magnetic deflection of, 48, 60
— Making paths of visible, 165
/3-rays, Definition of, 139
Barium, 15, 85, 214
Barkla, C. G., 238
Becquerel, Henri, 6, 7, 128
Beryllium, 214
Bismuth, 15, 146, 148, 191, 214,
233
Boltwood, B., 125, 133
Bonds of affinity, 213
Boron, 205, 214
— Atomic weight of, 248
Bragg, Sir William, 34, 35, 45, 62,
63, 64, 220, 235
Branch Series, 201
Breviiun, 150, 214
Broeck, van der, 231
Bromine, 214
— Atomic weight of, 248
Bunsen, K. W.,75
7-rays, 29-32, 66, 237
— Radiograph by, 31
Cadmium, 214
Caesium, 75, 214, 239
Calcium, 214
— absorption of gases by, 52, 101
Carbon, 106, 213, 214, 224
— Atomic weight of, 248
Carnotite, 20, 98
Cascade of changes, 74
Cathode-rays, 52-58, 210, 245
Cause of atomic disintegration, 114
Cerium, 214
Chance of disintegration. 111
Change, Law of radioactive, 112
— of radio-elements, 71, 74, 91,
92 el seq.
Chemical combination. Nature of,
216
— elements, bonds of affinity, 213
Number of, 239
Order of, 212
Table of, 214, 231, 239
Chemists and radioactivity, 109
Chlorine, 105, 213, 214, 215
— atomic weight, 248
Chlorion, 213
Chromium, 214
Cloud method of making paths of
rays visible, 64
Cobalt, 214, 240
Conservation of radioactivity, 88
Constancy of radioactivity, 10, 13,
23, 24, 27, 43, 69, 70, 77, 90, 172
Control of natural energy, 5, 13,
173, 184
Copper, 214
Corpuscular theory of radiation,
38
Cosmical aspect of life, 179
— energy, 24, 120, 174, 178
Cost of scientific investigations, 19
Cranston, J. A., 204
Crookes, Sir William, 15, 42, 52,
57, 128, 165
Crookes' tubes, 52, 58, 210
Crystal, space-lattice, 235
Curie, M. and Mme., 10, 12, 13,
15,19, 75,83, 124, 136,145, 198
D
" Dg" line, 97, 100, 101
Dalton, John, 106, 108, 217
Debierne, A., 99, 198, 203
Decay of radioactivity, 70, 87
Definition of the atom, 105-108
Detection of infinitesimal quan-
tities, 17, 75, 77, 82, 85, 90, 91,
95, 109
Determination of average life, 115
Dewar, Sir James, 52
Diffraction grating, 234
INDEX
255
Discovery of radioactivity, 6
Discrete theory of radium rays,
40, 44
Disintegration, see Atomic dis-
integration
— , Chance of, 111
— series, Analogies between, 188-
191, 198
of actinium, 186, 198-204
of thorium, 178, 186-198
of uranium, 121-151
Doctrine of energy, 20, 27, 37,
68, 178, 185
Dysprosium, 214
E
" E-ray," 204
Earth, Age of the, 25, 75, 177-183
— Internal heat of, 178
Earthquake routes, 180
Effects of radioactivity, 8-11, 28
Eka-tantalum or proto-actinium,
204
Electric current, Action of magnet
on, 49
Electricity, Discharge of, 8, 14, 18,
34, 45, 64, 211
— Nature of, 50
Electrolytic dissociation. Theory
of, 213-217
Electro-magnet, 47
Electro -magnetic inertia, 211
Electrometer, 46
Electron theory of matter, 109,
212
Electrons, 55-58, 63, 109, 210, 212,
216, 247
— period of revolution, 236
— valency, 217, 249
Electroscope, Gold-leaf, 8, 17, 42,
59, 84
Electrostatic and electromagnetic
deflection methods, 55, 57, 60,
225, 245, 247
Elements, Chemical, bonds of
affinity, 213
Number of, 239
Order of, 212
Stability of, 72, 157, 163
Table of, 214, 231, 239
Unchanging character of, 72,
73, 163, 227
— Isotopic, 229, 231
— Rare-earth, 218
— Rarity of, 155
Elixir of life, 182
Emanation of radium, 68, 77-94,
214
— a-particles from, 79, 145
Emanation of radium. Atomic
weight of, 85, 103
— Average life of, 113, 116, 122
— Chemical nature of, 84, 105
— Condensation of, 80-82
— Density of, 85
— Heat generated by, 85, 86, 170
— Physiological action of, 82
— Rate of decay of, 87, 88
— Reproduction of, 88, 89, 90j
122
— Spectrum of, 85
— Volume of, 82, 119
— of actinium, 198, 203
— of thorium, 136, 190, 193-198
Emanations and radiations con-
trasted, 78
Emanium, 203
Energy, cosmical. Source of, 174
— Doctrine of, 20, 27, 37, 68, 178,
185
— Internal, of matter, 68, 71-73,
86, 87, 91, 96, 108, 168-176
— Measurement of, 22, 71
Energy of coal, 22, 23, 70, 120
— of radioactive substances, 3, 5,
10, 58, 62, 68, 91, 92, 172
— of radium, 22, 68, 86, 119, 171
— of uranium, 170-172
— Transformers of, 69, 90
Ephemeral transition-forms, 74,
92, 116, 121, 129, 203
Equilibrium, Radioactive, 90, 95,
117, 196
Ether, The, 37, 38, 56
Erbium, 214
Europiiun, 214
Evolution of elements, 134, 162,
163
— of universe, 26, 120, 175
Existence, Struggle for, 6, 184
F
Facts and theories of radio-
activity, 89, 108
Fajans, K., 229
Faraday, Michael, 47, 55
Fleck, Alexander, 228
Fletcher, A. L., 166
Fluorescence, 6, 18, 31, 53, 66,
78, 79, 195
Fluorine, 214
— Atomic weight of, 248
Friedrich, M., 235
G
Gadolinium, 214
Gallium, 205, 214
256
INDEX
Gas, A radioactive, 77, 80, 83
Gases, bombarded by a-particles,
224
Geiger, Dr., 45
Geological bearing of radioac-
tivity, 26, 75, 175-180
Geology, Controversy between
physics and, 26
Germanium, 205, 214
Giesel, F. 0., 20, 99, 203, 204
Gold, 214, 231, 233, 239
— currency, 156
H
H-particles, 223, 224, 225
Hahn, Otto, 188, 205
Halogen family, 218
Halos, Pleochroic, 165
Hartley, H. B., 242
Heat generated by radium, 18, 19,
22, 85, 119, 178
in the earth, 178
Heaviside, Oliver, 210
Helium, 44, 60, 84, 94-104, 209,
214
— atomic number, 231
weight, 245, 248
— Discovery of, 97
— Liquefaction of, 97
— Possible isotope of, 225
— Prediction concerning the
origin of, 98
— Production of, by radium, 94,
99
by actinium, 99, 102
by thorium and uranium, 100
— in radioactive minerals, 96, 97
Volume of, 98
— Spectrum of, 99
Hersch3ll, Sir John, 159, 162
High vacua, 50, 52
Hitchins, Miss A. F., 130, 134
Holmium, 214
Homogeneous Characteristic X-
rays, 238
HSnigschmid, O., 243
Huggins, Sir William, 107
Hydrogen, 107, 214, 224, 289
— atomic number, 231
weight, 248
Incandescent age, 179
— gas-mantle, 14, 187
Increase of activity of radixim
with time, 16, 155
Indifference of radium to its en-
vironment, 27, 77
Indium, 214
" Induced radioactivity," 136
Inertia, 56, 211
Infinitesimal quantities. Detection
of, 75, 76, 82, 85, 90, 91, 95, 109
Inglis,J. K.H., 113
Integral values of atomic weights,
248
Intermediate substances, 74, 76,
77, 131-135
Internal energy of matter, 68, 70,
71-73, 86, 87, 168
— heat of earth, 178
Interpenetration of atoms, 63
Iodine, 214, 239
lonisation of gases, 8, 63, 64, 66
— of liquids, 215
Ionium, 133, 151, 153, 154, 164,
189, 205
— atomic weight, 243
— Average life of, 134
— Estimated period of, 134, 165
— and uranium X, Connection
between, 133
Iridium, 214, 239
Iron, 106, 214
Isotopes, 133, 150, 160, 229, 231-
233, 240, 248
— Separation of, 243, 248
Joachimsthal mine, 15, 152
Joly, John, 166, 176-178
— "Radioactivity and geology,
177
K
K-Series of X-rays, 238
Kalgurli, mines at, 132
Katrine, Loch, 125, 131
Kelvin, Lord, 20, 37, 178
Kirchoff, 75
Knipping, P., 235
Krypton, 214
L-Series of X-rays, 238
Lanthanum, 198, 214, 218
Laue, M., 235
Law of proportionality, 118, 123,
152
— of radioactive change. 111
Lead, 214, 231
— Atomic weight of, 241
Lead and radiiun, Connection be-
tween, 15, 76, 148, 241
— and thorium. Connection be-
tween, 191, 241
Life from the cosmical standpoint,
179
INDEX
257
Life of radio-elements, 92
— Period of average, 113
Light, Nature of, 36, 39
— Velocity of, 38, 58, 211
Limitations of knowledge, 4, 6,
66, 173, 178-180, 227
Lithium, 214
— atomic number, 231
Lutecium, 214
M
M-Series of X-rays, 238
Macdonald laboratories of M'Gill
University, 89
Mackenzie, T. D.,130
Magnesium, 214
Magnetic deflection of cathode-
rays, 53
Maintenance of radium, 121
sun's energy, 24, 120, 179
Manganese, 214, 239
Marckwald, W., 146, 147, 192
Marsden, E., 223
Mass of the electron, 55-57, 210
Matter, Electron theory of, 109
— Ultimate structure of, 209
— Unsolved problems of, 109,
206
Maxwell, J. Clerk, 158, 162, 181
McCoy, H. N., 125
Measurement of energy, 22, 71
Meitner, Miss L., 205
Mendelejeff, D., 205
Mental pictures, 109
Mercury, 85, 148, 214, 231
— atomic weight, 247
Merion, T. R., 242
Mesothoriima, 187-193, 227
Metaneon, 244
— atomic weight, 248
Mica, Coloration of by a-rays, 166
Milngavie, reservoir at, 125, 131
Minerals, Helium, in radioactive
96,97
— Lead in radioactive, 15, 148
— Quantity of radium in, 16, 75,
123, 152
— Ratio between quantities of
uraniiun and its products in,
152
Minimum quantity of helium de-
tectable, 101
radium detectable, 17, 42
Molecules, 2, 108, 158
Molybdenum, 214
Monazite sand, 186, 192
Moseley, H. G. J., 239, 249
Multiple atomic disintegration, 200
N
N-particles, 224, 225
Negative and positive electricity,
50
Neodymium, 214
Neon, 84, 214, 245
— atomic weight, 245, 248
Newton, Sir Isaac, 38
Nickel, 214, 240
Niobium, 214
Nitrogen, 213, 214, 224
— Atomic weight, 248
"Niton," 78
Nomenclature concerning atoms
and molecules, 106-108
Non - separable radio - elements,
154, 187, 195, 227
Nuclear atom, 61, 210, 220
O
O-partieles, 224, 225
Onnes, K., 97
Osmium, 214, 239
Ouroboros, 181
Oxygen, 106, 214, 224
— Atomic weight, 243
P-3 route of earthquakes, 180
Palladium, 214, 239
Parent of ionium, 133
— of radium, 122-134
Penetration test of rays, 7, 29, 30,
31, 80
Period of average life, 113
connection with range
of a-rays, 164
— half change, 115
Periodic law, 105, 171, 205, 212,
227-229
— table of the chemical elements,
214, 228, 231, 239
Perpetual motion, 21, 24, 59
Phosphorescence, see Fluores-
cence
Phosphorus, 214
— Atomic weight of, 248
Photographic effects of radio-
activity, 8, 14, 18, 66, 80
Physical impossibility, 25
Pitchblende, 15, 75, 127, 152,
187, 243
Planet, niunber of revolutions, 236
Platino-cyanides, 31, 35, 141
Platinum, 214, 239
Pleochroic halos, 165
Polonium, 16,44, 146, 154,199,214
258
INDEX
Positive and negative electricity,
50
— rays, 245, 246
Potassium, 105, 214, 240
Praseodymium, 214
Prediction of origin of helium,
98
Proportionality, Law of, 118, 123,
152
Proto-actinium, 205
Q
Quantity of helium detectable by
spectroscope, 101
in minerals, 98
— of radium in minerals, 16, 75,
124-127, 152
R
Radiant matter, 52, 57, 211
Radiation, Nature of, 36-39
Radiations, Complex, 28
Radioactivity, a new science, 1
— discovery, 6, 26, 175
— Four experimental effects of, 8
— an unalterable atomic pro-
perty, 12
Radiograph by 7-rays, 31
Radio-tellurium, 146, 148
Radio-thorium, 187-196, 227
Radivun and uranium, connection
between, 124-127
— Active deposit of, 137
— Average life of, 117, 125
— Changes of, 136
— Chemical nature of, 15
— clock, 59, 249
— Cost of, 19
— A changing element, 73
— emanation. See Emanation of
Radirnn
— Experiments with, 18
— Growth of, 134
— Maintenance of, 121-135
— " physically impossible," 26
— Quantity of, in pitchblende,
16
— Radiations from, 139
— Reproduction of, 122
— series, 207
— Substitute for, 154, 187, 193
— War uses of, 19
Radium A, 89, 105, 139-144, 153
— B, 89, 105, 139-144
— C, 105, 139-144, 151, 164, 201
— C, 151, 202
— D, 153, 190
— D, E, and F, 145-147
Radium F, Identity of, with
polonium, 147
Ramsay, Sir William, 78, 82, 84,
85, 97, 99, 119, 188, 223
Ratio between uranium and its
products, 152
Rayleigh, Lord, 59, 84
Rays of radioactive substances, 9,
28 et seq.
Recoil, Radioactive, 103, 104
Recovery of radioactivity of
radiiun, 77, 88
Rhodium, 214, 239
Ronigen, Wilhelm K., discovery of
X-rays, 6
Rowland diffraction grating, 234
Rowland, Professor, 160
Royds, T., 102
Rubidium, 214
Russell, A. S., 229
Ruthenimn, 214, 239
Rutherford, Sir Ernest, 29, 30, 45,
46, 60, 78, 80, 86, 89, 98, 102,
119, 125, 136, 161, 188, 197, 220,
222, 223, 224, 225, 231
Samarium, 214
Scandimn, 205, 214
Scattering of a-particles, 63, 222
Schuster, Arthur, 161
Selenium, 214
Self-induction, 211
Sidot's hexagonal blende, 36
Silicon, 205, 214
— Atomic weight of, 248
Silk tassel experiment, 18, 34
Silver, 214, 239
Simplon Tunnel, Radium in rocks
of, 177
Sodion, 215
Sodium, 214, 215, 238
Solar systems, compared and con-
trasted with atoms, 226
Spectra of isotopes, 242
Spectroscope, 75, 91, 92, 97, 99,
101, 129, 160, 163, 179, 209
Spinthariscope, 42, 44, 65
Stability of elements, 72, 157, 163
Standard, The International
radium, 17
Strontivun, 214
Struggle for existence, 6, 184,
Strutt, Hon. R. J. (now Lord
Rayleigh), 59, 125, 176
Substitute for radium, 154, 187, 193
Successive changes of radio ele-
ments, 74, 77, 89, 110, 116,
129-133, 138, 145-149
INDEX
259
Sulphur, 214
— Atomic weight of, 248
Sun's energy. Maintenance of, 24,
120, 178-180
Synthesis of atoms, 180, 204
Table of atomic weights of " pure "
and " mixed " elements
(Aston), 248
disintegration series com-
plete, 207
periods and quantities, uran-
ium series, 153
velocities and ranges of
a-rays, m-anium series,
162
— Periodic, of the elements, 214
Chart showing sequence of
a- and j3-changes through,
230
Tantalum, 205, 214, 219
Tait, Professor, Recent Advances
in Physical Science, 20, 25, 26
Tellurium, 214, 240
Terbium, 214
Thallium, 148, 214, 231, 233
Theories and facts of radio-
activity, 89, 108
Thomson, Sir Joseph, 55, 57, 210,
212, 245, 246
Thorium, 13, 94, 97, 98, 100,
102, 133, 136, 154, 186-198,
214
— Active deposit of, 194-198
— atomic weight, 243
— halos, 166
— disintegration series, 178, 186-
198, 207, 227, 229
— Production of helirnn from,
100
— Ultimate product of, 190, 242
Thorium A, 190, 197
— B, C, D, 164, 190, 201
— C, 201
— Emanation, 190, 193-196
— X, 190, 195
Thulium, 214
Tin, 214, 238
Titanimn, 214, 218
Total energy in radium, 119
in uranium, 170-172
Transcendental character of radio-
activity, 27, 58
Transformers of energy, 69
Transmutation, 13, 71, 72, 172,
182, 209, 223-225, 233, 248-250
Tungsten, 214
U
Ultimate product of thorium, 190,
24
— products of radium, 76, 96, 123,
147, 148, 242
Ultra-material velocities, 63, 221
Unchanging character of elements,
72, 73, 163
Unsolved problem of matter, 109,
206
Uranium, 7, 12, 97, 98, 102, 107,
116, 124, 148, 169-172, 188,
189, 194, 207, 214, 229, 239
— atomic nrnnber, 214, 231
— Average life of, 116, 125
— halos, 166
— Production of helium from,
100
— and radium. Connection be-
tween, 124-134
— I and II, 149, 165, 189, 195
206, 227
— Y, 205
— X, 128-131, 133, 150, 188, 206,
214
— XiandXg, 150
and ionium. Connection be-
tween, 133
Vacua, High, 50, 52, 249
Valency electrons, 217, 249
Value of gold, physical explanation
to accoiuit for the unchang-
ing, 156
— of radium, 19, 156
Vanadium, 214
Velocities, Ultra-material, 63
Velocity of cathode-ray particle,
58
— of light, 38, 58, 211
Visible, Making the paths of rays,
64
Volume of helium in minerals,
98
— emanation in equilibrium with
radium, 82, 119
W
Wave-length of 7-rays, 237
— of X-rays, 234-238
Wave theory of light, 39
Welsbach, Auer von, 13, 243
Whytlaw-Gray, R., 85
Willemite, 35, 53, 78, 79, 81,
87
260 INDEX
Wilson, C. T. B., 64 ♦
Writing by radium, 19
X
X-rays, 6, 30, 31, 38, 78, 210, 234
— Diffraction of, 234
— wave-length, 234-238
Xg gas, 245
Xenon, 214
— Atomic weight of, 248
Ytterbium, 214
Yttrium, 214
Zero family, 217, 245
Zinc, 214, 238
— sulphide, 35, 80,
197, 203, 204, 223
Zirconium, 214, 239
141, 196
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